The present work investigates experimentally continuous spray fluidized bed layering with external product separation. In this process configuration, growing particles are continuously withdrawn and classified externally by two sieves into product fraction, undersize and oversize fraction. The oversize fraction is milled and fed back to the fluidized bed alongside with the undersized fraction. New seed particles for the layering process are provided by recycling of milled particles and agglomeration of dust particles, formed by dried droplets (overspray) and attrition. Until now, this process configuration has been investigated by simulations only. For certain parameter regimes, these models predict oscillations of the particle size distribution. Based on these predictions, even theoretical controllers have already been developed to diminish these oscillations. An experimental investigation of these oscillations of the particle size has not yet been published. Our work presents a systematic study of the process and proves the existence of oscillations of the particle size distribution in the fluidized bed. (C) 2015 Elsevier Ltd. All rights reserved.