Thin magnetic films of Co‐P were autocatalytically deposited on two different plastic substrates. A polycarbonate and a polyimide plastic were chosen as the nonconductive substrates because of their thermal stability and flatness. The conditioning, sensitization, activation, and autocatalytic deposition of these nonconductors are described. A citrate hypophosphite bath was used with variations in the ratio of cobalt salts to citrate salts. The magnetic properties (coercive force, remanence, and squareness) of the thin (250–4000Aå) films were analyzed, and the most promising concentration ratio and substrate were chosen for further experimentation. Variations in reductant concentrations were made at a constant ratio of cobalt to citrate. For each concentration of reductant, films of different thicknesses were prepared, and the effects of composition and thickness on the crystallographic and magnetic properties of the deposits were studied.