102 lakes distributed along the Catalan Pyrenees (from 0°41′ E to 1°48′ E) were sampled during summer in 1987. The alkalinity of the lakes was low (<300 μequiv l⁻¹) but there was no regional acidification. Most chemical variance was related to the nature of the bedrock. Lakes on Devonian, Cambro-Ordovician, Silurian or granodioritic batholites were easily distinguished. Even between different batholites, there were significant differences. There was no clear distinction either in the chemistry or in trophic status between disturbed and undisturbed lakes, except for certain extreme cases. Pasture is the most important source of eutrophication, but is restricted to small, very shallow and usually endorreic lakes.