Investigation of the conversion of acetic acid (a model oxygenate) on well-defined metal and metal phosphide surfaces provides insight to the reasons for the high selectivities of transition metal phosphide (TMP) catalysts for hydrodeoxygenation of bio-oils. Temperature programmed reaction (TPR) of acetic acid isotopologues on pristine Ru(0001) and a P modified Ru(0001) surface, representative of the Ru2P (111) facet, show an identical sequence of elementary steps for acetic acid decomposition. The inclusion of P atoms in the surface does, however, lead to electronic changes of the Ru atoms and increase activation barriers for C-O, C-C, and C-H bond scission by values of similar to 5-10 kJ mol(-1). The differences increase the prevalence of C-C bond rupture over C-O bond rupture by a factor of two (determined in TPR measurements). Based on the changes in product selectivity and the relative changes in activation energies for C-C and C-O bond rupture, these results indicate that the addition of P atoms will increase selectivities toward C-O bond rupture during high temperature, hydrotreating catalysis, which matches reported HDO selectivities that are greater on TMP catalysts than on the corresponding metallic nanopartidies. These results provide fundamental insight to the reasons for the greater selectivity for C-O bond rupture. (C) 2017 Elsevier Inc. All rights reserved.