When dealing with gas-solid reactions in rotary kilns, it is necessary to realize that the total particle surface within the granular bed can be much larger than the outer surface of the bed. Depending on the reaction conditions this inner surface can contribute considerably to the chemical conversion in the kiln. In this paper, a model is presented, which describes the reaction within the bed for the case of bed movement according to the cascade mode. In this case, gas is drawn into the rotating bed together with the particles. As a key quantity, an effectiveness factor eta of the bed is defined. It is the ratio of the actual conversion to the conversion that would occur if the concentration of the reacting component remained unchanged throughout the bed, i.e. at its entrance concentration. An evaluation for reactions of order m shows this factor to be more than 25 % when the Damkohler number is smaller than 2. It approaches 100 % as the Damkohler number approaches 0. The Damkohler number used in this paper contains the void fraction of the particle bed in its denominator.