Causes of episodic stream acidification and acid-neutralizing capacity (ANC) declines were investigated in five streams on forested, unglaciated catchments on the northern Appalachian Plateau of Pennsylvania, during October 1988 to April 1990. Increased SO4 concentrations during episodes were the primary cause of ANC declines on three of the streams. Episodes on the remaining two streams were dominated by base cation dilution and/or SO4 increases. Base cation dilution was most important in a stream with initially high preepisode ANC caused by seeps from limestone outcrops. Bedrock layers on this basin were dipping toward the channel, which promoted more rapid movement of low-ANC subsurface storm flow water to the channel. A stream with even higher preepisode ANC, but on a basin with geologic structure that shunted low-ANC subsurface storm flow away from the channel, exhibited only minor base cation dilution. Organic acidity contributed to ANC decreases on all five streams, although it was a minor source compared with SO4 and dilution. NO3 and Cl did not contribute to ANC decreases on any of the study streams. Aluminum increases acted to reduce episodic ANC declines especially in the more acidic streams. Episodic acidification on the northern Appalachian Plateau in Pennsylvania was similar to that found in mid-Atlantic streams where SO4 increases to play a dominant role, but unlike glaciated regions in the Northeast where NO3 can be important. ¿ American Geophysical Union 1994