[en] The chemical and isotopic compositions of rainwater were monitored at Los Azufres geothermal field (88 MWe) and its surroundings during May-September 1995, which is the rainy season. Samples were collected from eight sites: three within the field, three in its surroundings and two sufficiently far from the field such that they have no geothermal input. The concentrations of Cl^-, SO₄^2- and NO₃^- were measured in about 350 samples and found to be generally <5 ppm. Chloride concentrations remained constant with time, but sulfate and nitrate concentrations decreased, which suggests a nearby industrial source for the sulfate and nitrate. A mixing model for Cl^-, SO₄^2- and delta³⁴S also suggests an industrial source for the rainwater sulfur. The determination of pH was found to be necessary, but is not sufficient to characterize rainwater acidity. The Gran titration method was used to determine alkalinity with respect to equivalence point of H₂CO₃^*. Values of alkalinity were found to range from 10^-4 to 10^-6 eq/L, and were negative only for some samples from Vivero and Guadalajara. Thus, SO₄^2- and NO₃^- are in general not in acidic form (i.e. balanced by Na⁺ Ca²⁺ etc. rather than H⁺). Sulfate delta³⁴S values were about -1.5 per mille in Los Azufres and its surroundings, and in Morelia, but differed from the value of -0.2 per mille for Guadalajara. The delta³⁴S values for H₂S from the Los Azufres geothermal wells are in the range -3.4 to 0.0 per mille. Thedelta³⁴S ranges for the natural and anthropogenic sources for environmental sulfur overlap, making it difficult to differentiate between the contribution of different sources. However, a similarity of values of delta³⁴S at Los Azufres and Morelia (85km distant) suggest a regional source of sulfate that is not associated with geothermal emissions from Los Azufres. (Author)