Changes in concentrations of H2S with distance away from the Olkaria geothermal power station, and at one discharging well were continuously monitored over periods of up to II days. The changes in H2S were correlated with changes in weather parameters. Results show that within the power station, concentrations of H2S can be expected to exceed 7.5 ppm over half hour periods at least once over a two week period. The levels of H2S decay with distance away from the power plant to levels below 0.3 ppm within about 0.5 km from the power station. Concentrations of H2S capable of causing plant injury are therefore expected to be present within 0.5 km of the power station, and over distances of several hundred meters away from a discharging well. The highest concentrations of H2S were obtained when winds were calm, humidity high, and atmospheric temperatures low. In the Olkaria area, these weather conditions exist between 2000Hrs and 0300 Hrs local time. The study suggests that if these weather conditions persist for prolonged periods of time H2S concentrations around the power station mayrise to unsafe levels with regard to human health. There is therefore need for an H2S monitoring program, and installation of an early warning system, particularly when weather conditions point towards the build up of H2S. Measurement of the acidity of rain water showed no evidence that the high H2S concentrations are contributing to acid rain formation in the immediate vicinity of the power station.