[C-13]Methanol and [C-12]propene (fed as isopropanol, which is immediately converted to propene) have been co-reacted over SAPO-34 in a flow system at 400 degrees C using argon as a carrier gas. The feed was equimolar in C-13 and C-12 atoms, The products were analyzed by gas chromatography-mass spectrometry, allowing determination of the isotopic composition. While the methanol was completely or almost completely converted to hydrocarbons, the larger part of the propene emerged unreacted. The products ethene and butenes were mostly formed from methanol and contained a large excess of C-13 atoms, The propene effluent consisted mainly of all-C-12 or all-C-13 molecules and, only to a small extent, isotopically mixed molecules. The tendency for propene to emerge unreacted and all new hydrocarbons to be formed from methanol became more pronounced with progressing catalyst deactivation. The results show that the higher hydrocarbons are, over this catalyst, not formed by successive methylations of bulk gas-phase propene. A previously proposed "carbon pool" mechanism can explain the gross effects seen in the product and isotopic distribution, but it is pointed out that the nonreactivity of propene in SAPO-34 may be caused by slow diffusion of propene in the pores.