The loss rate of H2SO4 vapor onto submicron particles was measured for three different particle substrates. The experimental technique involved direct flow tube measurements of H2SO4 decay rates onto a polydisperse aerosol using chemical ionization mass spectroscopic detection. The aerosols of this study were partially hydrated crystalline salts with diameters in the size range of 20 to 400 nm. The mass accommodation coefficients, α, were calculated from the first-order rate constants for H2SO4 loss to be 0.73¿0.21 and 0.79¿0.23 for loss onto (NH4)2SO4 and NaCl, respectively. Measurements of the loss rate of H2SO4 onto a NaCl aerosol coated with stearic acid resulted in lower mass accommodation coefficients with values of 0.31 and 0.19 for aerosol with high and low stearic acid coverage, respectively. The observed decrease in α on an aerosol with a hydrocarbon coating suggests that aerosol composition is a key factor in H2SO4 adsorption on to a particle surface.¿ 1997 American Geophysical Union