The concern over the environmental effects of acidic rain has increased the interest in understanding the processes which control the acidity of cloud and rainwater1-6. In regions affected by anthropogenic emissions, H2SO4 and HNO3 are most often responsible for lowering the pH of rain below 5.6, the value water attains in equilibrium with atmospheric CO2 (ref. 7). In more remote regions, however, formic acid (HCOOH), and to a lesser extent, acetic acid (CH3COOH), have also been identified as major acidic components of rain8. These two organic acids have also been observed in the gas phase in the southwestern US9. While the sources of HNO3 and H2SO4 are at least qualitatively understood5,6,10-12, the sources of organic acids remain largely unknown. We have investigated the coupled gas- and aqueous-phase cloud chemistry of HCOOH and report here that during the daylight hours, aqueous-phase OH radical reactions can both produce and destroy HCOOH in cloud droplets and may, in fact, control the HCOOH levels in rain. Similar mechanisms may also exist for acetic and other organic acids.