Photocurrent multiplication ('photocurrent doubling') effects were studied at n-GaN electrodes in 1 M H2SO4 solutions containing formic acid. The photocurrent multiplication factor f was measured as a function of different parameters. It was found that f increases with increasing formic acid concentration, reaching a limiting value at high concentrations. Surprisingly, this limiting value for f was found to depend upon the extent of photoanodic etching prior to performing the photocurrent multiplication measurements: it increased from about 1.05 for 'new' electrodes to a maximum of 1.3 for etched electrodes. This suggests that somehow, intrinsic surface states are involved in the photo-oxidation of formic acid, the concentration of which changes due to etching. Further, it appeared that n-GaN dissolves photoanodically in 1 M H2SO4 solutions containing formic acid, even at very high concentrations. A mechanism, accounting for all of the experimental findings was proposed. © 2004 Elsevier Ltd. All rights reserved.