A kinetic study of cadmium and lead biosorption to a brown alga, Kjellmaniella crassiforia, was carried out. The shrinking core model derived by M. Gopala Rao and A. K. Gupta (Chem. Eng. J. 24,181, (1982)) was modified and adapted for description of the rate process of cadmium and lead biosorption to the alga. The biosorption rate process was well described and average apparent diffusion coefficient of about 9 x 10(-6) cm(2) s(-1) was found for both cadmium and lead ions. The value was 20 to 50 times higher than the apparent diffusion coefficients of cadmium and lead ions in strong-acid resins like Dowex 50W-X8.