A comparison of isostructural H-SAPO-34 and H-chabazite in high space-velocity experiments of the methanol to olefins (MTO) reaction has been performed. The initial selectivity was shown to be similar for both catalysts, and also to be independent of acid site density. However, the deactivation rate was critically dependent on the acid site density. Thus, low acid-site density samples exhibited longer lifetimes at low space velocity conditions than their high acid-site density analogues.