We have demonstrated that second-harmonic generation (SHG) can be used to investigate the in situ transfer of molecules between colloidal microparticles. The transfer kinetics of organic cations, malachite green (MG) initially adsorbed on to spherical polystyrene sulfate (PSS) microparticles, to disk-shaped clay particles in aqueous solution was obtained. The surface population of MG on the PSS particles during the transfer process can be measured in real time. In the present studies, the adsorption free energy and the number of adsorption sites were found from the adsorption isotherm measured by the SHG method (Wang, H.; Yan, E. C. Y.; Liu, Y.; Eisenthal, K. B. J. Phys. Chem. B 1998, 102, 4446). The kinetics of transfer was observed to depend linearly on the particle density of clay. The observed transfer was biexponential with decay times of 17 +/- 1 s and 217 +/- 4 s for the case of a clay particle density of 3 x 10(9)/cm(3). This method was further applied to study the transfer of MG between PSS microparticles. At a PSS density of 4 x 10(8)/cm(3), the decay time was 40 s. Because of the coherent nature of the SHG signal, the transfer kinetics can be obtained in situ by the SHG method. A kinetic model is used to describe the transfer process from which the desorption and adsorption rate constants of MG from and onto PSS particles were obtained.