In many forested headwater catchments, peak SO₄^{2 -} concentrations in stream water occur in the late summer or fall following drought potentially resulting in episodic stream acidification. The sources of highly elevated stream water SO₄^{2 -} concentrations were investigated in a first order stream at the Sleepers River watershed (Vermont, USA) after the particularly dry summer of 2001 using a combination of hydrological, chemical and isotopic approaches. Throughout the summer of 2001 SO₄^{2 -} concentrations in stream water doubled from ???130 to 270 ??eq/L while flows decreased. Simultaneously increasing Na⁺ and Ca²⁺ concentrations and ??³⁴S values increasing from +7??? towards those of bedrock S (???+10.5???) indicated that chemical weathering involving hydrolysis of silicates and oxidation of sulfide minerals in schists and phyllites was the cause for the initial increase in SO₄^{2 -} concentrations. During re-wetting of the watershed in late September and early October of 2001, increasing stream flows were accompanied by decreasing Na⁺ and Ca²⁺ concentrations, but SO₄^{2 -} concentrations continued to increase up to 568 ??eq/L, indicating that a major source of SO₄^{2 -} in addition to bedrock weathering contributed to peak SO₄^{2 -} concentrations. The further increase in SO₄^{2 -} concentrations coincided with an abrupt decrease of ??³⁴S values in stream water SO₄^{2 -} from maximum values near +10??? to minimum values near -3???. Soil investigations revealed that some C-horizons in the Spodsols of the watershed contained secondary sulfide minerals with ??³⁴S values near -22???. The shift to negative ??³⁴S values of stream water SO₄^{2 -} indicates that secondary sulfides in C-horizons were oxidized to SO₄^{2 -} during the particularly dry summer of 2001. The newly formed SO₄^{2 -} was transported to the streams during re-wetting of the watershed contributing ???60% of the SO₄^{2 -} during peak concentrations in the stream water. Thereafter, the contribution of SO₄^{2 -} from oxidation of secondary sulfides in C-horizons decreased rapidly and pedogenic SO₄^{2 -} reemerged as a dominant SO₄^{2 -} source in concert with decreasing SO₄^{2 -} concentrations in spring of 2002. The study provides evidence that a quantitative assessment of the sources of stream water SO₄^{2 -} in forested watersheds is possible by combining hydrological, chemical and isotopic techniques, provided that the isotopic compositions of all potential SO₄^{2 -} sources are distinctly different. ?? 2010 Elsevier Ltd.