The poisoning effect of carbon disulfide on cubooctahedral palladium nanoparticles (1.5, 3.0 or 4.6 nm in mean diameter) deposited on montmorillonite was investigated in the liquid-phase hydrogenation of styrene to ethylbenzene under mild conditions. The dispersion of the metal crystallites was computed as a function of the mean diameter in terms of face atoms, of edge atoms + corner atoms, and of the total number of exposed surface atoms. A linear relationship was found between the rate of hydrogenation and the fraction of low-coordination surface sites poisoned, independently of particle size, and hydrogenation ceased when these sites were quantitatively covered by poison molecules with a nominal stoichiometry of CS2:Pd = 1:2. High-coordination terrace sites are not involved in the hydrogenation reaction.