On a fairly filthy night in Llanelli, Ulster's hopes of furthering their interest in making the European quarter-finals for a fifth season in succession effectively came to an end after they failed to prevail in what was a must-win clash for both sides.
Despite mounting an admirable second half comeback, after trailing 12-0 at half-time, which saw Ulster score a try through Darren Cave and then land two Ruan Pienaar penalties to narrow the Scarlets' lead to just 15-13, they just couldn't get enough from what was a scrappy game to eclipse the home side - as they had managed in the first of these back-to-back encounters - and ended up coughing up a soft try in the dying minutes to even rob them of a losing bonus point.
Ulster also had to play 10 minutes in the second half while reduced to 14 men, after Declan Fitzpatrick was shown yellow for repeated scrummaging offences - an area where Ulster suffered numerous penalties on a desperately hard evening for the visiting eight - though they did manage to get within those two points of the Scarlets while down a man and had a great chance to score a try only for Cave's 66th minute pass to not reach Luke Marshall.
Pienaar also pulled a tricky 75th minute penalty wide which would have given Ulster the lead for the first time in the game just as their ranks were fully restored.
But that was as good as it got for Neil Doak's men and there would be no remarkable turnaround.
However, the real damage to Ulster's hopes had already been done in the first half when a string of penalties, poor plays and a wobbly scrum were punished by four Rhys Priestland penalties to give the Welsh side a more than useful cushion on the way to avenging their heavy defeat in Belfast in the first round of games between the sides.
The Scarlets opened the scoring after Liam Williams had put in an inch-perfect kick to Ulster's corner flag. Franco van der Merwe was wrapped up by George Earle -in for Jake Ball who had been downed by a virus - at the lineout and was penalised for not releasing; Priestland slotted the penalty to get things going after 12 minutes.
Then, just before 15 minutes were up, Scott Williams knocked on in his 22 from a low Rhodri Williams pass and Ulster had a great chance to attack but, on the second reset, referee JP Doyle pinged the visiting front row after the set-piece had caved in.
It looked like a harsh call and then after Priestland missed his penalty to touch, Robbie Diack knocked on and Ulster conceded another scrum penalty off Fitzpatrick.
Scott Williams then burst through between Luke Marshall and Ian Humphreys and though the pass failed to find the support the net result was yet another penalty against Ulster for preventing possession coming back to the Scarlets; Priestland duly made it 6-0 after 19 minutes.
This became 9-0 on 28 minutes after Ulster strayed offside as the Scarlets put some width on the ball and Priestland did the needful. Then, two minutes after the half hour, Ulster managed to snaffle a spilt ball and after trying to offload their way out of their own territory, Rory Best was, perhaps rather harshly, penalised for apparently not releasing.
Anyway, it all resulted in Priestland nailing his fourth penalty to take the scoreline to 12-0 after 32 minutes but he was unable to land his fifth attempt, which duly followed two minutes later, when Ulster were pinged at the breakdown.
Matters didn't improve for Neil Doak's men as, after a scrum caved in in the 38th minute, Priestland opted to take another shot at the sticks. However, it fell just short and that brought an end to the opening 40 minutes with the home side leading 12-0.
Getting the first score seemed absolutely fundamental for Ulster after the sides returned to the pitch and a minute in Darren Cave combined with Luke Marshall and took the return pass to score under the sticks.
Humphreys converted and at 12-7 it was game on again.
After Declan Fitzpatrick was warned by the referee over his binding, Priestland was wide with another penalty.
Ulster then got another break after Johan Snyman hit Humphreys late but, after their second lineout maul was launched, Best and Clive Ross were pinged for breaking off and taking out defenders. So, yet again, Ulster were undone by sloppy play.
Their cause was further rocked when Humphreys pulled his first penalty wide after the Scarlets infringed at the breakdown.
But the home side began to get the upper hand again and the result was another penalty given against Luke Marshall, after a strong surge from the Scarlets. Priestland's kick came in off the upright to take the Welsh side's lead to 15-7.
It got worse for the visitors three minutes after the hour when Fitzpatrick was binned after another collapsed scrum. But 14-man Ulster did come back and after Bowe hit a great angle, and Cave juggled his pass, the Scarlets were penalised. Pienaar stepped up to kick the three points and cut the home side's lead to five with 10 minutes to play.
The South African did it again two minutes later with a huge effort just making it over the posts and Ulster now trailed by just two points.
Ulster then took the play down into the Scarlets' 22 and from a lineout maul they got a chance to go for goal to take the lead but Pienaar's 75th minute effort drifted just wide.
But the bitterest pill came with two minutes to go. James Davies gratefully intercepted a laboured pass off the back of a scrum from Roger Wilson aimed at Pienaar and the Scarlets flanker dashed in unopposed to plant the ball under the sticks.
Priestland drop-kicked the conversion - and that was that.