Formic and acetic acid were measured in the gas phase at three sites in eastern and northern Austria using an annular diffusion denuder sampling technique. The daytime background concentration of formic acid averaged 1.4 ±0.2 ppb (63 ± 9 nmol m-3) in a semirural area in eastern Austria. In a rural area in Austria the corresponding level was 0.9 ±0.3 ppb (40 ± 13 nmol m-3). About twice as high levels of formic acid were found during a photochemically active summer period and a wintertime pollution episode. The concentration of acetic acid was generally about 50 % lower than that of formic acid during daylight hours. In summer diurnal variation with a night-time minimum and a daytime maximum in the late afternoon hours was observed. The diurnal variation was not evident in results from measurements above the inversion layer or during winter conditions with snow cover. No increase in the formic and acetic acid levels was found in the airshed of Vienna, indicating that traffic emissions do not form a major primary source for the semirural concentrations measured. Although there might be biogenic sources for formic and acetic acid, our findings suggest a noticeable contribution from various anthropogenic emissions being a source for photochemical production of the acids.