In addition to decreases in base cations associated with increased discharge, four other factors contribute to episodic depressions in pH and acid neutralizing capacity (ANC) in low-order streams in Maine: (1) increases in NO3 concentrations; (2) increases in organic acidity; (3) increases in anion fraction of SO4 concentrations; and (4) salt-effect generated acidity from soil solutions. In conjunction with increased discharge, factors 1, 2, or 4 appear necessary to depress pH to less than 5. The chemistry of individual precipitation events is irrelevant to the generation of acidic episodes, except those caused by high loading of neutral salts in coastal regions. Increases in discharge, but not necessarily in dilution of solutes, in combination with the chronically high SO4 from atmospheric deposition, provide the antecedent chemical conditions for episodic acidification. Differences in antecedent moisture conditions determine the processes that control output of either ANC or acidifying agents to aquatic systems.