A model for the chemical reduction of platinum tetramine in perfluorinated Nafion(R) membranes using sodium borohydride as reducer is proposed. A Nernst-Planck equation is employed for the description of ion transport by diffusion and migration and a reaction term accounts for the in situ chemical reduction. Time-dependent concentrations of the diffusing species within the membrane are obtained numerically by an iterative technique until completion of the precipitation. The model assumes that mass transport is limited by diffusion and migration within the membrane and that the concentrations remain constant at the interfaces during the precipitation. The model shows the effect of (i) the reducer concentration in the solution, (ii) the number of precipitation cycles and (iii) the rate of chemical reduction. To check the validity of the model, metallic platinum concentration profiles across the membrane thickness are obtained by electron microprobe analysis. Values of the diffusion coefficients of the diffusing species within the membrane are obtained from conductivity and permeation measurements.