The effect of CH2Cl2 impurities on the conversion of CH3Cl to olefins over SAPO-34 has been investigated. CH3Cl was co-reacted with CH2Cl2 in a 10:1 ratio at 400 degrees C, WHSV = 8 h(-1), p(CH3Cl) = 620 mbar and p(CH2Cl2) = 65 mbar, over a typical SAPO-34 catalyst with (Al + P)/Si = 11. It was observed that CH2Cl2 promotes the formation of aromatic compounds and leads to rapid deactivation of the catalyst for the CH3Cl to olefins reaction. The induction period, which is typically observed as an increase in activity before maximum conversion is reached is, however, significantly shortened by the addition of CH2Cl2. Feeding a mixture of (CH3Cl)-C-12 and (CH2Cl2)-C-13 in a 10:1 ratio led to formation of olefins with about 5% C-13, which was indistinguishable from the C-13 content in the aromatic hydrocarbons (reaction intermediates and coke) retained inside the SAPO-34 crystals in the same experiment. This shows that CH2Cl2 takes part in the reactions leading to both products, including the formation of aromatic reaction intermediates, and deactivation. Hypothesized reaction steps for the incorporation of carbon atoms from CH2Cl2 into both gas phase products and coke were found to be plausible by studying the reactions of xylene/CH2Cl2 mixtures and of 2,5-dimethylbenzyl chloride alone. The intimate mechanistic links between the desired conversion of CH3Cl to olefins and the detrimental effects of CH2Cl2 addition imposes restrictions on the purity of the CH3Cl feedstock in a commercial SAPO-34 based CH3Cl to olefins application. (C) 2009 Elsevier B.V. All rights reserved.