The kinetics for the sorption of molybdate and phosphate by four Chilean Andisols have been determined. About 55% of the molybdate and 61% of the phosphate was sorbed in the first 0.5 h, after which sorption slowly increased, reaching 90% for molybdate and 97% for phosphate after 72 h. At the same time, OH -ions were released into the external solution, raising its pH by 0.85 units for molybdate and by 0.65 units in the case of phosphate. These observations indicated that both anions were sorpbed by a ligand exchange mechanims. Among the five kinetic models examined (Table, 2), the Elovich equation gave the best fit of the experimental data (R 2 = 0.93 to 0.97, standard error = 0.35 to 0.94). The sorption rate constant ( ) for both anions was related to the organic matter (