Abstract: As part of a cooperative study with the Geologic Division of the U.S. Geological Survey, water samples collected from geothermal power production wells at Cerro Prieto, Mexico, were analyzed for selected elements by d.c. argon plasma emission spectroscopy. Spectral interferences due to the presence of high concentrations of Ca, Si, Na, and K in these waters affected the apparent concentration values obtained. These effects were evaluated and correction techniques were developed and applied to the analytical values. Precipitates present in the samples at the time of analysis adversely affected the accuracy, precision and interpretability of the data.