Three mass transfer models are developed for the vapor phase. The first model assumes a rigid interface between the liquid and vapor and yields the Schmidt number as the correlating variable. The other two models represent a free interface with the penetration theory as the mechanism for one model and eddy diffusion as the mechanism for the other model. Both free interface models correlate experimental data within the error of the data. A liquid-phase mass transfer model is developed for a free interface and the penetration theory as the mass transfer mechanism. Design equations are developed from this analysis.