The dehydrative conversion of methanol-to-hydrocarbons over acidic zeolite catalysts has been extensively studied, showing that different classes of hydrocarbons, such as olefins and gasoline-range hydrocarbons, can be formed as a result of the shape selective nature of zeolites, although these reactions often proceed with low selectivity. It is now widely accepted that the conversion of methanol/DME proceeds through a "hydrocarbon pool" mechanism, in which organic molecules present in the zeolite pores act as co-catalysts for olefin formation. In this work, DME was reacted over H-ZSM-5, using propene and toluene as co-feeds to determine the effect of the hydrocarbon pool composition on product selectivity. Co-feeding DME with small amounts of propene (<10 C%) resulted in a product mixture rich in C 4-C 7 aliphatics (47 C%) whereas co-feeding DME with toluene resulted in a product selectivity dominated by C 2-C 3 olefins (36 C%) and aromatics (45 C%). The reaction of DME with a mixed propene/toluene co-feed resulted in an intermediate product selectivity, compared to the single co-feed cases. Varying the olefin:aromatic ratio in the co-feed allowed for the product selectivity to be varied predictably and systematically. The trends did not change for reactions with less than 25 C% conversion. These results show that product selectivity of DME reactions over zeolites can be tuned by varying the composition of the hydrocarbon pool, and thus controlling the relative rates of olefin and aromatic methylation. The effect of temperature, DME pressure, and zeolite crystal size on the hydrocarbon pool evolution and product selectivity will also be discussed.