The decomposition of methanol on Pt nanoclusters grown from vapor deposition onto an ordered Al2O3/NiAl(100) thin film was investigated under ultrahigh vacuum conditions with various surface probe techniques. The Pt clusters had mean diameter near 2.3 nm and height 0.4 nm before their coalescence; consisting of phase fcc, the clusters grew with their facets either (111) or (001) parallel to the θ-Al2O 3(100) surface, depending on the temperature of growth. More than half the adsorbed monolayer of methanol on the Pt clusters decomposed via two channels: dehydrogenation to CO and C-O bond scission. The dehydrogenation was dominant and induced first at low-coordinated Pt sites, at 150 K on Pt(001) clusters and 200 K on Pt(111) clusters, whereas both low-coordinated and some terrace Pt sites exhibited reactivity, despite the cluster size. On average, one CO was produced per surface Pt site, for a monolayer of methanol on either Pt(111) or Pt(001) clusters. In the other reaction, scission of the C-O bond occurred primarily in methanol itself and began about 250 K; the intermediate methyl preferentially formed methane on combining with atomic H from dehydrogenated methanol. No preferential reaction site for the C-O bond scission is indicated, but this process showed a remarkable dependence on the size and lattice parameter of the clusters: the probability of C-O bond scission decreased when the size increased and the lattice parameter decreased.