Carbonate ions in the aqueous electrolyte solution improved both the photocurrent and the stability of a porous BiVO4 semiconductor electrode used for water splitting under visible light up to 520 nm. The photocurrents in AHCO(3) (A = Na and K) solutions at 1.23 V were approximately six times higher than that in Na2SO4 aqueous solution. The maximum incident photon-to-current efficiency (IPCE) was 45% at 420 nm without any surface modifications.