Microkinetic models based on first principles calculations have been developed for the vapor and liquid phase hydrodeoxygenation of propionic acid over a Pt (1 1 1) surface. Calculations suggest that decarboxylation does not occur at an appreciable rate. In the vapor phase, decarbonylation products, propanal and propanol are all produced at similar rates. However, in both liquid water and 1,4-dioxane, propanol and propanal are favored over decarbonylation products. While a condensed phase can shift the reaction rate and selectivity significantly, the dominant pathways towards the various products are hardly affected. Only for propanal production do we observe a shift in mechanism. At 473 K, the propionic acid conversion rate is increased by one order of magnitude in liquid 1,4-dioxane relative to the gas phase. In liquid water, the conversion rate is similar to the vapor phase since adsorbed propionic acid blocks a large fraction of the surface sites. (C) 2019 Elsevier Inc. All rights reserved.