A model of surface renewal is developed in this application for the coating of pharmaceutical tablets in rotary drums. The theoretical description of the model is based on a population balance for the processed particles, resulting in,a system of partial differential equations. The tablet bed is divided in two separated domains: - where particles are dried and mixed (bulk of the bed), - where particles are coated by spray atomization of an aqueous solution of polymer (feed zone). The feed zone is located close to the surface of the tablet cascade and its size is determined by the surface impact of the spray. The model is completely determined by three parameters: the size of the feed zone, the rate of coating solution and the turnover time of the tablets in the bed. Experiments have been carried out in a batch pilot rotary drum. The results indicate that the volume of the feed zone depends on the rotational speed and the drum filling. The turnover time essentially depends on the rotational speed in the range of the studied operational conditions. The model shows,that the coating mass distribution can be roughly approximated by a normal distribution centered around the mean coating mass. The model predictions are in agreement with the experimental results obtained in an industrial batch drum.