a b s t r a c t Formaldehyde and acetaldehyde are two most abundant carbonyls in ambient air. Biogenic emission has been proposed as a significant source other than anthropogenic emissions and atmospheric secondary formation. Here at a forest site in South China, the carbon isotopic compositions of formaldehyde and acetaldehyde emitted from leaves of three tree species (Litsea rotundifolia, Canarium album and Castanea henryi) were measured in comparison with the bulk carbon isotopic compositions of tree leaves. d 13 C data of the emitted aldehydes (from À31& to À46&) were quite different for tree species, which were all more depleted in 13 C than the tree-leaf bulk d 13 C values (from À27& to À32&). Formaldehyde in ambient air at the forest site had d 13 C values different from those of leaf-emitted formaldehyde, indi-cating other sources for ambient formaldehyde apart from direct emission from leaves, most probably the photooxidation of biogenic hydrocarbon like isoprene and monoterpene. The d 13 C differences of acetaldehyde between ambient data and those of tree leaves emission were less than 1&, implying direct biogenic emission as the dominant source.