Individual rain events were sampled above and below the forest canopy in Walker Branch Watershed, Tennessee, from August 1977 to June 1978 for the purpose of analysis of acidity and organic content. Strong acid content correlated with sulfate concentrations. Total acid concentrations were conserved as rain penetrated chestnut oak (Quercus prinus L.) canopies, although the strong/weak acid ratio declined substantially. Although weak acids constituted at least 30% of the total acidity of incident rain and increased to over 50% in throughfall, the increase could not be entirely attributed to organic acids. The results of the study support the fundamental importance of ion-exchange models in systematizing rain-vegetation interactions.