We have studied the kinetics of methanol steam reforming on a commercial CuO/ZnO/Al2O3 catalyst in a packed bed reactor. The objective of this work was to assess the impact of deviations from isothermality in packed bed reactors on the rates of methanol steam reforming. Preliminary experiments with catalyst dilution showed higher apparent rate constants as the catalyst was diluted, implying heat transfer limitations in the bed. We therefore varied the reactor diameter from 4.1 mm down to 1 mm in order to improve heat transfer. The smaller diameter reactor showed higher apparent catalyst activity. This increased reactivity could be explained using a 2D pseudo-homogeneous reactor model. The heat transfer limitations result in a temperature gradient of up to 40 K in the 4.1 mm reactor. Even the I mm reactor suffers from temperature variations of up to 22 K. Since packed bed reactors are most commonly used for the production of H-2 via steam reforming, it is important to recognize the role of these heat transfer limitations. Transport limitations can lead to falsified kinetics and lowered catalyst productivity. (c) 2004 Elsevier B.V. All rights reserved.