Silver nanoparticles were formed electrochemically in the presence of poly(N-vinylpyrrolidone) (PVP) to generate highly stable nanoparticles with a mean diameter of 12 nm. The optimised conditions involved formation at - 6.0 V vs Ag|Ag+ with a PVP:Ag+ ratio of 75:1. Particle agglomeration was observed when the PVP:Ag+ ratio was reduced to 25:1 and when extended polarisation periods were employed. Using the optimised conditions, the silver nanoparticles remained stable for periods in excess of 2 months. The nanoparticles were immobilised inside a polyacrylamide hydrogel and used for the reduction of 4-nitrophenol. The rate constant obtained for the reduction of 4-nitrophenol was calculated as 7.1 x 10(-4) to 1.6 x 10(-3) s(-1) depending on the amount of silver. The hydrogel-Ag composite was stable and could be stored for several months, while there was little change in the rate constant when it was repeatedly used.