Use of light gas by-products in the production of paraxylene by the methylation of toluene and or benzene

A process for producing paraxylene by the catalytic alkylation of benzene and/or toluene with methanol, which produces a para-rich mixture of xylene isomers, together with water and some light organic by-products, particularly dimethyl ether and C4− olefins. The off-gas stream, containing the C4− olefins, may be recycled back to the reaction to be co-injected with methanol to reduce the methanol self-decomposition and the reaction of methanol to olefins or to fluidize catalyst particles recovered by a reactor cyclone. By using recycled off-gas rather than water or steam, the deleterious effects of water and/or steam on the catalyst aging and activity rates and the size of downstream equipment necessary to recover olefin by-products may be reduced.