A series of selective hydrogenation reactions, cinnamaldehyde to cinnamyl alcohol, crotonaldehyde to crotyl alcohol, citronellal to citronellol, and o-chloronitrobenzene to o-chloroaniline, were studied over polymer-stabilized nanoscale noble metal colloid catalysts in solutions. Metal cations and metal complex ions significantly affected the catalytic performances of the noble metal colloidal nanocatalysts. Some metal cations increased both the activity and the selectivity of the nanosized catalysts, and metal complex ions exhibited more complicate effects. (c) 2005 Elsevier B.V. All rights reserved.