Gaseous acids can be emitted as well as taken up by plant foliage, but little is known about the influence of environmental factors on the exchange process. In a laboratory study we investigated the short-term effects of temperature and light on the exchange of acetic and formic acids between orange foliage and the atmosphere. The results were compared with diurnal exchange cycles obtained with the same species under field conditions. In the field, the exchange of volatile acids showed pronounced diurnal variations with maximum emissions of about 0.15 nmol m-2 projected leaf area s-1 for acetic acid and 0.3 nmol m-2 s-1 for formic acid during noon and afternoon and with acid deposition of up to 0.1 nmol m-2 s-1 for both acids during early morning and night. Under laboratory conditions no significant acid deposition (<0.01 nmol m-2 s-1) could be observed, and emission rates were lower than in the field and ranged around 0.035 nmol m-2 s-1 (SD = 0.013) for acetic acid and about 0.08 nmol m-2 s-1 for formic acid (SD = 0.033) at 30 °C and 1000 μmol m-2 s-1 PAR. A clear positive response of acid release to light could be observed. Emissions were very low in darkness and strongly increased with light up to 1000 μmol m-2 s-1 PAR. The emission response to light occurred within our hour and was also observed with other acids emitting plant species investigated. Response of emission to temperature was more variable, though, on the whole, the effect was also positive. Finally, we observed a diurnal rhythm in acid release which was not related to other measured climatic or physiological parameters. We conclude that in the short-term the exchange of volatile acids between leaves and air under natural conditions is stimulated by light and, somewhat, by temperature, while a leaf-internal process promotes acid release in the morning and higher acid concentrations in ambient air promote acid deposition.