Alexis Sanchez scored the only goal of the game midway through the second-half when he tapped home Neymar's pinpoint cross to score for the third consecutive game yesterday.
However, it was another frustrating evening for Lionel Messi as he failed to score for a fourth consecutive La Liga match.
Barca now lead second-placed Atletico Madrid by four points and Real Madrid by nine with both sides from the capital having played a game less.
However, the toll of five games in 13 days appeared to affect the Spanish champions as their build-up play was laboured in the first-half and they caused Espanyol very few problems.
Kiko Casilla was forced into an early save from Alexis Sanchez after nice work by Iniesta and Messi.
But it was the visitors who had the best chance of the half when Victor Sanchez sneaked in behind Dani Alves and Victor Valdes had to race from his line to turn the midfielder's effort behind.