In this study, size-controllable gold nanoparticles (NPs) are synthesized by using sonoelectrochemical methods in the presence of natural chitosan (Ch). First, positively charged Au-containing complexes were prepared in 0.1 N NaCl by using oxidation-reduction cycle (ORC) treatment to Au substrate. Then size-controllable Au NPs, ranging from 10 to 80 nm in diameter, were synthesized by varying the addition quantities of Ch from 1 to 2 g/L, respectively, in solutions under using sonoelectrochemical reduction. Experimental results indicate that the optimal molar ratio of used Ch to synthesized Au NPs is ca. 3 for obtaining the smallest Au NPs. Moreover, the catalytic activity of the size-controllable Au NPs on the decomposition of acetaldehyde in solution was examined. Au NPs with 10 nm in diameter demonstrate significantly catalytic activity. However, bigger Au NPs with 80 nm in diameter are almost inert for the decomposition of acetaldehyde. (C) 2012 Elsevier Ltd. All rights reserved.