Despite resting several star names David Villa, Andres Iniesta and Cesc Fabregas were on target to continue the Catalan giants majestic start to the campaign which has seen them only drop points in La Liga with their draw in the El Clasico with Real Madrid.
Lionel Messi, Xavi Hernandez and Pedro Rodriguez were among the players left at home in Barcelona but it made little difference as the visitors were always in control against an Alaves side which has seen better times.
The 2001 UEFA Cup finalists who lost to Liverpool are now in Segunda B, the Spanish third tier.
The Alaves keeper Urtzi Iturrioz was kept busy by an Alexis Sanchez header and a shot by Iniesta from the edge of the area before Villa fired Barcelona ahead on 40 minutes as he crashed a shot in off the crossbar after receiving a long ball from Iniesta.
The roles were reversed five minutes after the restart as Villa set up Iniesta for a curling 20-yard strike which nestled in the top corner.
Fabregas ensured a comfortable second leg at the Nou Camp with a header two minutes from the end.
Earlier Valencia overcame the rain and Segunda B Llagostera’s artificial pitch for a 2-0 victory.
Valencia have had an inconsistent start to the season but they came out on top with goals from Jonas Goncalves at the end of the first half and Nelson Valdez after 68 minutes.
Llagostera held their own in a battling performance and Valencia barely deserved their lead from Jonas who failed to connect cleanly with a volley but it sneaked in the corner.
Valdez headed them further ahead and Llagostera wasted the chance to reduce the arrears with Marc Sellares missing from the penalty spot late on.