The kinetics have been studied of the reaction between p-Me2NC6H4NH2 (DMPPD) and Co(NH3)(5)Cl2+, a process of relevance in color photography, in a water/AOT/n-heptane microemulsion catalyzed by nanoparticles of colloidal palladium. The rate of formation of the colored N,N-dimethyl-p-semiquinonediimine (Sf) was found to be first order in Co(NH3)(5)Cl2+ and almost zero order in DMPPD and to increase linearly with the concentration of palladium. Studies over the 10-45 degrees C temperature range gave activation energies which increased markedly as the temperature fell. Voltammograms of the oxidation of DMPPD and the reduction of Co(NH3)(5)Cl2+ using a rotating Pd electrode were carried out to investigate the catalytic mechanism. Evidence was obtained of DMPPD adsorption on the metal surface which indicated that the rate-determining step in the catalysis was diffusion of Co(NH3)(5)Cl2+ through a layer of DMPPD adsorbed on the palladium particles in the water pools.