use of a droplet train apparatus, the mass accommodation coefficients (alpha) of gas-phase HCl, HBr, HI, and CH3COOH were measured for I-octanol to probe the nature of the hydrophobic organic surface as a function of relative humidity and temperature (263-283 K). In the absence of water vapor, alpha for both HBr(g) and HI(g) is unity, independent of temperature. The mass accommodation coefficients for acetic acid and HCl are smaller, about 0.3 for acetic acid and 0.01 for HCl at 273 K, displaying negative temperature dependence. The value of alpha for acetic acid is independent of relative humidity. However, values of a for HBr, HI, and HCl change dramatically as a function of relative humidity. As the relative humidity increases, the alpha values for HBr and HI decrease, and alpha for HCl increases. At a relative humidity of about 50%, alpha for all three species converges to that on pure water. A model is proposed to explain these unexpected results.