The chemical composition of rainfall in northeast Uruguay in 1999 and 2000 and the sources that contribute to the rainwater chemistry are assessed in this study, contributing to a limited knowledge base of rainwater quality in South America. Principal factor analysis and cluster analysis indicate that four main source groups influence the rainfall chemistry over a range of spatial scales. Terrigenous sources (e.g. rock, soil and dust) and agricultural sources (e.g. livestock and crop fertilization) contribute to rainwater chemistry on a local and sub-regional scale. Influx of marine air from the Atlantic Ocean has a regional-scale influence while biomass burning contributes at both a local and subregional scale (e.g. fuel wood, and agroindustries) and through long-range transport (forest fires and land clearing). As may be expected in this area dominated by agriculture, the concentrations of ions that are indicative of industrial emissions, NO 3 À and SO 4 2À , are typical of background measurements. The current data are limited, but provide an indication of rain quality and the sources that influence its chemistry in Uruguay. It may be inferred that large-scale biomass burning in the central parts of the continent influence rainwater chemistry on a scale far larger than indicated by the Uruguayan network. Similarly, but to lesser extent, the influx of marine air off the Atlantic Ocean has a greater regional-scale influence than suggested by these data. r