Despite progress in theoretical tools, the influence of solvation in heterogeneous catalysis remains poorly understood and predicted due to the large computational burden. In this work, we show that the inclusion of the solvation by water using a continuum model thermodynamically inhibits the O–H bond scissions involved in the ethanol aqueous phase reforming reaction over Pt(111), while it tends to favor the C–H, C–C, and C–O scissions. Then, we present a novel group additivity scheme for the free energy of adsorbates at the Pt/water interface that is able to capture this solvent effect. The mean absolute error (MeanAE) for the Gibbs free energy of formation is 3.3 kcal/mol over the investigated set of 200 species at the interface and the MeanAE for the 151 reaction free energies of ethanol aqueous phase reforming is 2.8 kcal/mol. Regarding the effect of solvation, our scheme is able to predict it with a MeanAE of about 1 kcal/mol. Together, the scheme promises to be accurate enough for narrowing down the most important reaction pathways in complex reaction networks as encountered in biomass conversion.