From the shape of stationary profiles of temperatures and moisture for the gas and solid phases, two distinct zones clearly appears in a counter-current cascading rotary dryer of sugar; a first zone in which the drying takes place, a second zone in which the dried sugar is reheated by contact with the entering hot air. The Bt of experimental data of temperature and humidities was used to test a mathematical model for the drying zone and to find optimal values of heat and mass transfer coefficients. Additional investigations at laboratory scale have provided information on the kinetics of drying of sugar crystals falling in a curtain as in the airborne phase of a rotary dryer. Sorption isotherms show that a strong re-moistening or deliquescence of sugar occurs at about 80% relative humidity. Obviously such a value must not be reached in an industrial dryer.