In this work, we report a new sonoelectrochemical pathway to prepare enriched and small Au nanoparticles (NPs) in aqueous solutions via aid of chitosan (Ch). First, Au substrate was cycled in a deoxygenated aqueous solution containing 0.1 M NaCl and 1 g/L Ch from -0.28 to +1.22V vs Ag/AgCl at 500 mV/s for 500 scans. The durations at the cathodic and anodic vertices are 10 and 5s. respectively. Subsequently, a cathodic overpotential of 0.6 V and an anodic overpotential of 0.2 V from open circuit potential (OCP) were applied in turn under sonication to prepare enriched and small Au NPs. The ratio of reaction time during cathodic overpotential to that during anodic overpotential was investigated to be 0.5 for obtaining enriched and smaller Au NPs of 5 nm in diameter in solution. The catalytic activity of the prepared Au NPs was examined by analyzing the decomposition of acetaldehyde in solution. Experimental results indicate that the decomposition of acetaldehyde based on the prepared Au NPs is significantly increased by ca. 240% of magnitude, as compared with that based on previously prepared Au NPs of 10 nm at the same Au concentration. (C) 2011 Elsevier Ltd. All rights reserved.