The effect of mean metal crystallite size on activity and selectivity of Pt/Al2O3 catalysts under conditions of ethanol steam reforming at low temperatures was investigated. It was found that catalytic activity is strongly influenced by metal crystallite size. Normalized turnover frequency (TOF divided by the length of the perimeter of the metal-support interface) increases by three orders of magnitude with increasing platinum crystallite size from 0.9 to 16.8 nm. Reaction is initiated by dehydrogenation of ethanol over the Pt surface, a reaction which is significantly enhanced over large Pt crystallites. Evidence is provided that the active sites for ethanol/adsorption dehydrogenation are platinum terrace sites of multiple Pt atoms, and that this may be the key step for ethanol reforming at low temperatures. (C) 2015 Elsevier B.V. All rights reserved.