The occurrence of episodic acidification in Canadian streams, lake waters and shallow groundwaters has been reviewed, and the contolling mechanisms identified. Episodes, which are periods of depressed alkalinity during hydrological events, have been studied mainly in southeastern Canada, and occur at all sites where there is sufficient time resolution of the observations, viz. Ontario, Quebec and Nova Scotia. An alkaline episode, where acidity decreases during an event, has been reported from one lake in the Canadian Artic. There is a bias towards the examination of episodes stimulated by snowmelt or rain-on-snow, since rainfall-stimulated episodes are poorly documented. Pre-event, rather than event, water dominates runoff during episodes. For this reason, biogeochemical reactions and the hydrological flowpaths in operation through the vadose and saturated zones are the principal controls on the chemical characteristics of episodes. Most episodes are dominated by base cation dilution in circumneutral systems, and increase in strong acid anions (particularly sulphate) in acidic systems. Episodes dominated by nitrification or organic acids or stimulated by sea salt input are rare or have not been decumented. Direct input of event water may dominate only during particular circumstances at snowmelt. Then, direct chemical inputs from lake ice and lake snow cover may be of importance in some systems.