The effect of intracrystalline diffusion on the conversion of methanol to olefins (MTO) over SAPO-34 of crystal sizes in the range 0.25-2.5 mm has been investigated at 425Β°C in a TEOM (tapered element oscillating microbalance) reactor. The 0.25 mm crystals resulted in a relatively large amount of dimethylether (DME) escaping the pores of SAPO-34 before being converted to olefins, hence giving lower olefin yields. The 0.4-0.5 mm crystals gave the largest capacity of olefin formation. The 2.5 mm crystals resulted in diffusion limitation in methanol conversion, with lower olefin yields as a result. The coking rates increased with increasing crystal size at low coke contents, levelling off at a lower level for the largest crystals than for the smaller crystals at high coke contents. The selectivity to different olefins was independent of the crystal size at relatively low coke contents. The selectivity on the fresh catalysts was controlled kinetically, and an effect of transition-state shape selectivity on the selectivity to olefins was induced by coke. The positive effect of coke on the ethene formation was explained in terms of a transition-state shape selectivity mechanism, as the formation of olefins with different sizes went through differently sized intermediates and the reactions with larger intermediates deactivated more quickly.