The product distribution and deactivation in the methanol-to-olefin (MTO) process over a phosphorous modified catalyst containing 10% HZSM-5 was studied in small diameter fixed bed reactors. These studies suggest that methane is formed directly from methanol and/or dimethyl ether and that non-aromatic C5+ hydrocarbons are intermediates in the MTO reaction, forming light olefins (C3(=) and C4(=), but not ethylene) by secondary cracking reactions. Based on photographs of the catalyst taken during the course of the reaction, three distinct coking patterns were observed that might be attributable to the different reactions. Deactivation time of the catalyst is highly dependent on the contact time, doubling the feed rate decreased the deactivation time by a factor of 10 and lowered the olefin production. Changing the feed from pure methanol to 10% methanol in nitrogen reduced methanol capacity of the catalyst considerably, but a slight increase in propylene selectivity was also observed. (c) 2007 Elsevier Ltd. All rights reserved.