The spatial distribution of lake-water pH near the Horne smelter in Rouyn-Noranda, Quebec, Canada, is affected not only by industrial sulphur dioxide emissions, but also by other anthropogenic and natural factors. Regional calcareous glaciolacustrine deposits from glacial lakes Barlow and Ojibway provide buffering capacity. Locally, some kettle lakes are buffered by silicate weathering. Small mines, tailings, and natural sources of acidity (e.g. peat basins, forested catchment areas with acidic soils) also contribute to lake acidification. Interpreting the modern regional distribution of lake-water pH, ranging from 3.7 to 9.3 in 99 lakes, is equivocal in this area where wind-transported emissions and buffering from calcareous glaciolacustrine deposits can yield similar spatial trends. However, in a parallel study, diatom assemblages have been used as bio-indicators of past lake-water pH since they respond to temporal shifts in environmental conditions, including lake-water acidity. The historical pH reconstructions for two lakes within a 100-km radius of Rouyn-Noranda suggest that they were naturally acidic before industrialization and have further acidified. Lac de la Pépinière was naturally acidic in the 1800s (pH 5.5) and reached a pH of 4.8 by 1998; its increased rate of acidification since 1927 corresponds to the beginning of mining and smelting operations.