Published correlations for the local efficiency of cross-flow distillation plates give widely different values of the liquid-phase resistance, though for most distillations the major resistance is in the gas phase. The simple penetration theory with equal contact times for the liquid and the gas indicates about 10% liquid resistance for distillation at one atmosphere. The penetration theory can be used to predict the increase in liquid resistance for distillation at high pressures and the low efficiencies for stripping operations.