While atmospheric aerosols are known to contain a significant fraction of organic substances, the influence of organics on the formation of H2SO4-H2O clusters and subsequent nucleation in the atmosphere is poorly understood. In the present Letter, bonding among formic and acetic acids, sulfuric acid, ammonia and water is studied using density functional theory at PW91PW91/6-311++G(3df,3pd) level. The stabilizing effect of formic and acetic acids is found to be close that of ammonia that indicates that the common organic substances may efficiently stabilize small H2SO4--H2O clusters and their involvement, alongside with or without ammonia, in the atmospheric nucleation should be studied further. (c) 2007 Elsevier B.V. All rights reserved.