A method-based on evaporation of a constant-composition droplet containing two components that differ markedly in volatility accurately estimates activity coefficients of both components. This new technique is developed to simultaneously determine the evaporation rate and composition of a droplet from intensity peaks observed in the light scattered by the droplet. It has no upper or lower limits on the relative volatility of the system and is particularly suitable for systems containing one relatively nonvolatile component. The data on the glycerol-ethanol system, obtained from evaporation of glycerol droplets in ethanol vapor and correlated with Margules and Wilson equations, are thermodynamically consistent. The results of this study agree with ethanol activity coefficients calculated from the fetal pressure vs. solution composition data previously reported.