In this paper we extend our investigations of selective hydrogenation reactions catalyzed by platinum-iron alloy surfaces from the determination of adsorption energies to the calculation of reaction pathways. Trying to elucidate the experimental fact that over Pt80Fe20 alloy surfaces alpha, beta-unsaturated aldehydes are selectively hydrogenated to unsaturated alcohols, we determine transition states and reaction rates for the hydrogenation of ethylene and formaldehyde to ethane and methanol, respectively, over pure Pt (111) and PtFe alloy surfaces. In a previous article (R. Hirschl et al., J. Catal. 217 (2003) 354) we argued that only iron atoms in the surface layer can explain the selectivity in hydrogenation. Here we investigate the influence of surface-iron atoms on energy barriers for hydrogenation. While Pt (111) and platinum-covered Pt80Fe20 (111) surfaces behave in a similar way, apart from reduced adsorption energies on the alloy surface, iron atoms in the surface have significant effects for the hydrogenation of formaldehyde. Although the reaction barrier is higher, the ratio of the energy barrier to the adsorption energy is much lower. Top-layer iron atoms do not change the behavior of the surface regarding the hydrogenation of ethylene. (C) 2004 Elsevier Inc. All rights reserved.