Coreaction of [C-13]methanol and ethene has been carried out over H-ZSM-5 (Si/Al = 45). The catalyst has very small crystals. Most experiments were carried out at 350 degreesC employing the partial pressures P-methanol = 50 mbar and P-ethene = 50 mbar. The reactor effluents were analyzed using gas chromatography. Isotopic analysis was carried out using GC-MS. A range of feed rates has been used, up to WHSV = 292 h(-1). This allowed extrapolation of results to zero contact time, giving information about the primary product distribution and the primary isotopic composition of the products. It also allowed the determination of the rate of ethene methylation by methanol. At very high feed rates the dominant coreaction product is propene (approaching 90%). At the highest feed rates the C-12(2) C-13(1) isotopomer constituted about 85% of the propene molecules. The reaction order for the methylation of ethene to form propene has been found to be one with respect to ethene and zero with respect to methanol. Measurements have been carried out over an extended range of temperatures, and an Arrhenius plot has been constructed. The apparent activation energy for the methylation of ethene was determined to be 10(9) kJ/mol. When corrected for the appropriate heat of adsorption for ethene, an intrinsic activation energy of 135 kJ/mol was found. Dimerization of ethene was insignificant under the investigated reaction conditions. Small amounts of aromatics (mainly xylenes) were always detected. These compounds were very rich in C-13, containing about 85% labeled carbons. (C) 2004 Elsevier Inc. All rights reserved.