Abstract In a previous paper [C. Duriez et al., Surf. Sci. 253 (1991) 190], molecular beam measurements were made of the adsorption-desorption kinetics of CO on Pd clusters, epitaxially grown on MgO(100). A large increase (∼8 kcal/mol) in the adsorption energy for clusters smaller than 3 nm grown on clean air-cleaved MgO was observed relative to clusters larger than 5 nm grown on UHV-cleaved MgO. New experimental measuremnts have shown that clusters smaller than 5 nm, grown on UHV-cleaved MgO or mica, give the same enhancement of reactivity as on a clean air-cleaved MgO surface, ruling out a possible substrate effect. The origin of this size effect is explained by the presence of a strong adsorption energy on some sites, the proportion of which increasing when the size of the clusters is decreased. Checks for carbon contamination by CO dissociation or CO disproportionation were negative.