As part of the Episodic Response Project (ERP), we intensively monitored discharge and stream chemistry of 13 streams located in the Northern Appalachian region of Pennsylvania and in the Catskill and Adirondack Mountains of New York from fall 1988 to spring 1990. The ERP clearly documented the occurrence of acidic episodes with minimum episodic pH ≤ 5 and inorganic monomeric Al (Al_im) concentrations > 150 μg/L in at least two study streams in each region. Several streams consistently experienced episodes with maximum Al_im concentrations >350 μg/L. Acid neutralizing capacity (ANC) depressions resulted from complex interactions of multiple ions. Base cation decreases often made the most important contributions to ANC depressions during episodes. Organic acid pulses were also important contributors to ANC depressions in the Adirondack streams, and to a lesser extent, in the Catskill and Pennsylvania streams. Nitrate concentrations were low in the Pennsylvania streams, whereas the Catskill and Adirondack study streams had high NO₃^- concentrations and large episodic pulses (≤ 54 μ eq/L). Most of the Pennsylvania study streams also frequently experienced episodic pulses of SO₄^2- (≤ 78 μ eq/L), whereas the Adirondack and Catskill streams did not. High baseline concentrations of SO₄^2- (all three study areas) and NO₃^- (Adirondacks and Catskills) reduced episodic minimum ANC, even when these ions did not change during episodes. The ion changes that controlled the most severe episodes (lowest minimum episodic ANC) differed from the ion changes most important to smaller, more frequent episodes. Pulses of NO₃^- (Catskills and Adirondacks), SO₄^2- (Pennsylvania), or organic acids became more important during major episodes. Overall, the behavior of streamwater SO₄2- and NO₃^- is an indicator that acidic deposition has contributed to the severity of episodes in the study streams.