Three hypotheses have been proposed to explain recent lake acidification in northwest Europe; the acid-deposition, the land-use and the long-term hypotheses. The hypothesis of natural, long-term soil acidification is not relevant for explaining recent, rapid lake acidification. A critical requirement in testing the first two hypotheses is to distinguish unambiguously between lake acidification due to atmospheric deposition and lake acidification following land-use and associated soil changes, such as the accumulation of raw humus. Hill-top lakes are small, deep basins perched on tops of cliffs and hills. They have very small catchments, often consisting of bare rock. The chemistry of these lakes is primarily influenced by the chemical composition of precipitation and by the underlying bedrock. Because of their very small catchments, their chemistry cannot be influenced by changes in catchment land-use. Hill-top lakes are thus ideal situations in which to test the two hypotheses, because any acidification in these lakes cannot be a result of changes in land-use, but only of acid precipitation. Thus if hill-top lakes have acidified recently, the land-use hypothesis would be falsified for these sites. On the other hand, the acid-deposition hypothesis would clearly be supported.