AbstractCopper colloids have been prepared by y‐radiolysis of cupric ions in the presence of poly(ethyleneimine). The reduction takes place in two, distinct steps. Copper (II) is reduced quantitatively to copper (I), and then nucleation of the colloid occurs after an induction period. The nucleation of colloidal copper is catalysed by the presence of less noble Pb(II) or T1(I) ions, which is explained by the generation of Pb and Tl nuclei in the solution, and their subsequent reoxidization by Cu(I). The formation of copper colloids is also catalysed by the presence of colloidal silver or silver ions, and radiolysis results in the formation of copper coated silver colloids. The optical shifts observed during the deposition of copper on silver cannot be explained by Mie theory. The differences are suggested to be due to weaker interband transitions in the copper monolayers than in the bu