Equilibrium relationships are defined between stream waters and weathering products, kaolinite and calcium montmorillonite, for the Rio Tanama system, west-central Puerto Rico. The major element composition of 46 water samples of springs and streams define a reaction path in the system CaO-Na 2O-MgO-Al 2O 3-SiO 2-H 2O between acid waters containing low concentrations of alkali cations and detrital reactant minerals. The principal reactant phases appear to be chlorite, plagioclase and orthoclase and occasionally anhydrite or calcite. Headward erosion by the Rio Tanama supplies the reactant phases to the stream silt load. The chemical denudation rate calculated for the Rio Tanama system is about 30 m/million yr. The chemical stream load appears to be buffered by the product phases in the main river over the 15-20 km river length sampled in this study. The silt and soil mineralogy and water compositions are used to define a log K for the hydrolysis of Ca-montmorillonite at 25°C of 35.0 ± 0.8. This value is in reasonable agreement with the value of 37.1 ± 1.0 defined by GARRELS and MACKENZIE (1967) in a similar manner for spring waters in the Sierra Nevada.