Heterogeneous reaction pathways involving water droplets in clouds and fogs are important mechanisms for chemical transformation of atmospheric trace gases. One of the fundamental parameters which determines the transfer rate of gases into aqueous droplets is the mass accommodation or ''sticking'' coefficient for gas molecules on the droplet surfaces. We have developed a laboratory method for directly determining mass accommodation coefficients (&ggr;) and have applied it to SO2 . The method combines a monodisperse droplet stream, generated at a vibrating orifice, with a low-pressure flow reactor. The uptake of the trace gas by the droplets is determined by measuring changes in gas phase concentrations, using tunable diode laser absorption, as the droplet stream is turned on and off. Results for SO2 give a value of &ggr;=(5.4¿0.6)¿10-2 at room temperature (295 K). However, this observed value for &ggr; is likely a lower limit to the true mass accommodation coefficient because of limitations imposed by the rate of gas phase diffusion of SO2 to the droplets and by the rate of hydrolysis of SO2 at the droplet surface. This measurement indicates that SO2 transport into clean aqueous cloud and fog droplets will not be limited by interfacial mass transport under normal atmospheric conditions. ¿American Geophysical Union 1987