Selectivity control is a major issue in chemical processes. In this work, isotopic switch experiments ((CH3OH)-C-12//(CH3OH)-C-13) were used to study the relationship between reaction intermediates and product distribution for the Methanol-to-Olefins (MTO) reaction over the archetype H-SAPO-34 catalyst during a full deactivation cycle. After switching, all alkenes contained a major fraction of C-13, and this fraction was only slightly influenced by catalyst deactivation. Among the methylbenzene intermediates, the C-13 fraction decreased with a decreasing number of methyl groups on the aromatic ring, throughout the test cycle. These observations clearly demonstrate that an observed increase in ethene selectivity with time on stream is due to product shape selectivity, which changes with the degree of pore clogging. Quantification of labeled and unlabeled methylbenzenes showed that less than 10% of the catalyst cages contained an active methylbenzene intermediate at any time during the test, even for 1 mu m crystal size. (C) 2009 Elsevier Inc. All rights reserved.