Strong empirical evidence links deposition of strong acids from the atmosphere to acidification of freshwaters and loss offish populations. Chemical processes in soils explain the composition of soil solution and surface waters and when coupled with input-output fluxes these processes, in concert with acid deposition, account for observed trends in soil and water acidification. The widespread recent acidification of surface water in eastern North America and Europe is due to the superposition of acid deposition upon natural acidifying processes in soils.