Frost formation in rotary heat and mass exchangers is investigated both experimentally and numerically. Experiments reveal that glazed frost, whose density and thermal conductivity are larger than that of rough frost, prevails in rotary exchangers operating in cold climates. A mathematical model for the prediction of the thermal behavior of the exchanger is presented. The model is validated with experimental data and employed to conduct a parametric study. Results indicate that the absolute humidity is the prevailing parameter to characterize the frosting phenomenon. A frost mass fraction chart is established in terms of the relative humidity of the warm exhaust stream and of the temperature of the cold supply stream. The transient three-dimensional model also shows that the absolute humidity and the temperature of both air flows vary nonlinearly in the frosted zone.