Solubilities and surface tensions were measured for difunctional organic acids in various concentrations of (NH4)2SO4 and NH4HSO4 aqueous solutions. Model results using these data indicate that the organic compounds affect cloud droplet growth by two mechanisms: by gradual dissolution in the growing droplet which affects the shape of the K¿hler growth curve, and by lowering of surface tension which decreases the critical supersaturation. ¿ American Geophysical Union 1996