The present study was undertaken to verify some of the assumptions involved in the modeling of Al geochemistry in Spodosols of the northeastern USA. Specific objectives were to determine if there is a mineral phase in the Bs horizons which regulates Al(3+) activities, evaluate the rate at which precipitation and dissolution reactions occur with the controlling mineral phase (referred to as Al(OH)3 precipitation hereafter, since the specific reactions controlling Al(3+) activity in soil solutions are unknown), and provide process-level information for the development of simulation models that depict Al retention/release and the acidification of drainage waters. The Bs horizon was chosen because it is the soil horizon thought to most influence surface water chemistry in these shallow Spodosols.