Based on the synthesis of light olefins from methanol (Methanol-to-Olefins, MTO) it will be shown that experimentally determined effectiveness factors of composite pellets of various sizes can be well described by the theory of SMIRNIOTIS and RUCKENSTEIN. The catalyst used consists of zeolite HZSM-5 particles embedded in an amorphous silica-alumina matrix. The effectiveness factor depends strongly not only on the pellet diameter and volume fraction of the active material but also on the reaction rate in both phases and the diffusion in the amorphous matrix. Consequently, the theory is an important mathematical tool for optimizing the performance of a composite catalyst by catalyst design.