Stable colloidal solutions of gold in water and ethanol were produced in the presence of small amounts of linear polyethyleneimine (PEI). In aqueous solutions a lowering of PEI concentration from 0.05 wt% to 0.0063 wt% results in an increase of the average size of Au particles from 3-7 nm to 30-40 nm. In ethanol 5-10 nm Au particles were produced in the presence of 0.0063-0.05 wt% PEI. At 0.05 wt% PEI the Au particles retain individual character but a lowering of PEI content to 0.0063 wt% causes particle aggregation into 80-140 nm globules. The Au PEI nanoparticles exhibit catalytic activity in reduction of methylviologen bication to cation-radical by sulfide-ions as well as a co-catalyst of photocatalytic water reduction by ethanol with the participation of mesoporous TiO2. In both cases individual 4-8-nm Au PEI particles show a much higher catalytic activity than 80-140-nm globules of aggregated Au PEI NPs synthesized at a lower PEI content. (C) 2014 Elsevier B.V. All rights reserved.