Pronounced and short-term changes in freshwater chemistry ('acidic episodes') can have a significant biological impact. More attention has been paid to chronic acidification, and there has been no previous attempt to produce a regional summary of occurrence of episodes in Europe, to describe their chemical characteristics, or to synthesize what is known of their controlling processes. These are the objectives of the review. The basic characteristics of episodes are explained, along with the problems of establishing an adequate description. Because of the relative paucity of other information on water composition, the review is based largely on measurements of short-term pH depressions, although other data are considered where available. Most of the studies reported here were conducted in Norway, Sweden, Scotland and Wales. (Copyright (c) 1992 - Elsevier Science Publishers B.V.)