The assessment of the effects of atmospheric inputs on the chemistry of mountain lakes requires realistic data on the chemical composition of these inputs. However, precipitation sampling stations are usually at lower altitudes than lakes. The location of the stations may lead to a mistaken appraisal of precipitation chemistry, since orography strongly influences the formation of storms caused by air masses coming from different directions, and altitude affects the mechanisms of transport and deposition of solutes in several ways. The effect of altitude on the chemistry of precipitation in the Pyrenees was studied at three sampling points located at different heights (1600, 1900, and 2200 m above sea level) within the same valley, orientated in a NE-SW direction. Sampling was carried out over a period (summer-autumn 1990) long enough to ensure a statistically representative collection of events with a different origin. Three main results were derived from this study. (1) The main source of variability was the cation-anion balance, which was determined by the origin of the wet air masses: rains from the Atlantic were more acidic than those coming from the Mediterranean. (2) There was no significant difference in the chemistry of bulk precipitation with altitude within the range 1600--2200 m asl, in which most of the lakes in the Pyrenees are found. (3) Below-cloud mechanisms of deposition had a significant influence on NO3- concentration. The concentration of most of the solutes was independent of precipitation volume, except for NO3-. ¿ American Geophysical Union 1996