The thermal decomposition of methanol has been studied on TiO2(110) as well as on Cu and oxygen-covered Cu nanoclusters supported on TiO2(110) using temperature programmed desorption (TPD). The sizes of the Cu clusters were characterized by scanning tunneling microscopy (STM). Methanol chemistry on the vacuum-annealed, reduced TiO2 surface itself produces ethylene as the main desorption product. Reoxidation of the TiO2 surface quenches the production of ethylene but also results in a new formaldehyde desorption peak at 870 K. The reactivity of methanol on small Cu nanoclusters (40.2 +/- 7.0 Angstrom diameter, 12.7 +/- 2.4 Angstrom height) is minimal, but trace amounts of formaldehyde, CO2, methane and H-2 are detected in TPD experiments, demonstrating that the Cu nanoclusters are more active than bulk single-crystal Cu surfaces. On oxygen-covered Cu nanoclusters, methanol reaction produces formaldehyde and CO2 as the major gaseous products as well as H-2, water, and methane. The yields of formaldehyde and CO2 increase as the Cu coverage is increased from 2 to 12 ML, indicating that these products are formed from reaction associated with the Cu surface. Lattice oxygen from the titania surface participates in methanol reaction because reoxidation of the titania surface with O-18(2) prior Cu deposition results in the evolution of (H2O)-O-18, (COO)-O-16-O-18, and (CO2)-O-18 in TPD experiments. No cluster size effects were observed for methanol chemistry on the pure Cu and oxygencovered Cu clusters.