The rate of n-butane hydrogenolysis on 1% Ru/Al2O3, the response of the rate to variation of H-2 pressure, and the product selectivities and their alteration with H-2 pressure, change with particle size and with the type of pretreatment used. The generality of particle size effects reported in the literature is rendered uncertain by these results; the predominant factor underlying them may be the strength of H-2 chemisorption.