Time-resolved measurements of the methanol synthesis reaction over a Cu/ZnO-based catalyst reveal a transient methanol production that depends on the pretreatment gas. Specifically, the methanol production initially peaks after a pretreatment with an intermediate mixture of H-2 and CO (20-80% H-2 in CO). The activity measurements are compared to Environmental Transmission Electron Microscopy (ETEM) observations of Cu nanoparticles supported on ZnO during exposure to comparable reaction conditions. The ETEM images reveal a gas-dependent morphology of the Cu nanoparticles with a marked flattening for a gas mixture of H-2:CO = 1:1. The gas-dependent morphology of the Cu nanoparticles provides a consistent explanation of the observed coupling between the transient methanol production and pretreatment conditions within the framework of the dynamic microkinetic model by Ovesen et al. [J. Catal. 168 (1997) 133-142]. (C) 2008 Elsevier Inc. All rights reserved.