Volatile organic compounds (VOCs) at the Sonnblick Observatory, Austria (3106 m a.s.l.) were measured in autumn 1999 using proton-transfer-reaction mass spectrometry (PTRMS). Anthropogenic VOCs at this remote site, such as benzene and toluene (50 pptv to 2 ppbv), were primarily traced to air from urban areas. In mid-November, following a hard freeze in the central Alps, a different pattern of VOCs developed similar to that seen with C5 and C6 leaf wound compounds (0.5 to 5 ppbv range). The common origins of these compounds are proposed to be freeze-damaged leaves, inferred from laboratory investigations that identified hexenals, hexanal, methylbutanals, pentenol and pentenone formation in such leaves. We suggest that deciduous forests can release large amounts of reactive VOCs during the autumn, and that these releases may be important for regional tropospheric chemistry. ¿ 2001 American Geophysical Union