Eustace's try, which came six minutes after half-time, proved decisive as although Ospreys fly-half Shaun Connor kicked his side back into the lead on the hour mark, his opposite number Ceiron Thomas hit back with a brace of penalties to steer the Scarlets to their first away win in the league in seven attempts.

The Ospreys' hopes were hit before kick-off when Brent Cockbain had to withdraw due to illness - Mike Collins deputised from the Welsh international at lock and number eight Filo Tiatia stood in as captain.

Thomas booted the visitors in front with a crisply hit third-minute penalty and Connor missed a chance to reply shortly after. There were a lack of try-scoring chances for the 10,143-strong crowd to chew on but a Thomas grubber kick almost saw the Scarlets cross the whitewash.

Thomas was wayward with a subsequent penalty kick and that allowed the Ospreys to have their first spell of dominance. Andrew Bishop speared through a midfield gap, darting into the Scarlets' 22, and while his pass was a loose one, Regan King's attempted clearance kick was charged down by Jonny Vaughton.

Ospreys winger Vaughton was sure he had the game's first try as he gobbled up the ball and dotted down, however Tim Hayes ruled the effort out as he adjudged that Bishop's earlier pass had drifted forward.

The Ospreys did turn around at the break with a 6-3 lead as Connor redeemed himself by landing two penalties, yet the Scarlets were back in front shortly after the resumption.

From an attacking situation, the Ospreys lost control of possession with Scarlets scrum half Gavin Cattle leathering the ball down-field. An error from the covering Stefan Terblanche, whose clearance kick flew horribly off the side of his boot, gave the visitors a lineout five metres from the Ospreys' line.

The try breakthrough came as despite some resolute defending from the hosts, Eustace managed to plough over from close range. Television match official Derek Bevan mulled over his decision when asked to confirm the grounding but the Scarlets got the thumbs up as they moved 8-6 in front.

Thomas missed the difficult conversion from the right and the Ospreys looked the more potent with both Vaughton and Bishop showing their class out wide. The latter set up a penalty which Connor duly sent through the uprights for a 9-8 scoreline.

Thomas replied for the Scarlets after Lee Byrne had caught Morgan Stoddart with a high tackle. Sensing a big finale, the Ospreys brought Gavin Henson off the bench for his first competitive appearance for the region since January but he had little influence.

The Ospreys should have dominated the closing stages as the Scarlets had replacement Darren Daniel sin-binned on 70 minutes for a dangerously late tackle on Byrne.

But Lyn Jones' side were left frustrated as the Scarlets pack had a magnetic hold on possession and a maul offence from the hosts offered Thomas the chance to stretch his side's lead. The fly-half's third successful kick of the night proved to be the match winner as the Scarlets ended a run of three successive defeats to their Welsh rivals.