C-13-Methanol and C-12 ethene (fed as ethanol) have been co-reacted over SAPO-34 in a flow system at 400 degrees C using argon as a carrier (diluent) gas. The feed contained an equal number of C-13 and C-12 atoms. The products were analyzed by CC-MS, allowing the determination of the isotopic composition of the reactor effluent. The ethanol was immediately converted to ethene, so the reaction system was equivalent to a feed consisting of methanol/ethene/water. While the methanol was completely or almost completely converted to hydrocarbons, the larger part of the ethene emerged unreacted. The products propene and butenes were mostly formed from methanol and contained a large excess of C-13 atoms. The ethene effluent consisted mainly of all C-12 or all C-13 atoms, and only to a small extent of C-12-C-13 molecules. The reaction system was followed from an initially very active catalyst until the catalyst was sufficiently deactivated that C-1 was not completely converted to hydrocarbons. The tendency for ethene to emerge unreacted, and for all new hydrocarbons to be formed from methanol became more pronounced with progressing catalyst deactivation. The results show clearly that the higher hydrocarbons are, over this catalyst, not formed by successive methylations of ethene. A previously proposed "carbon pool" mechanism can explain the gross effects seen in the product and isotopic distribution.