Commonly, catalytic reactions are carried out in batch reactors and analyses are performed upon reaction completion yielding little information about time-resolved concentrations of reactants and products. Herein an in situ analysis by real-time mass spectroscopy provides time-resolved information about heterogeneous catalytic reactions, such as reaction completion time and product formation. This paper presents a novel application of real-time mass spectrometry to monitor the progress of an alkylation reaction of benzene with propylene to produce cumene, using an H+ beta-zeolite (Si/Al: 12/1) catalyst at 150 degreesC with a 7:1 benzene to propylene molar ratio. Quantitative analysis of products was performed by gas chromatography (GC). Mass spectroscopy (MS) was utilized to continuously monitor reactants being consumed and products being formed during the alkylation process in real-time, to identify side products formed during the reaction, and to determine reaction completion. The real-time monitoring indicated that propylene was quickly absorbed/consumed, while cumene had a delayed release back into the gas phase. (C) 2003 Elsevier Science B.V. All rights reserved.