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The thermal decomposition of methanol has been studied on TiO 2 (110) as well as on Cu and oxygen-covered Cu nanoclusters supported on TiO 2 (110) using temperature programmed desorption (TPD). The sizes of the Cu clusters were characterized by scanning tunneling microscopy (STM). Methanol chemistry on the vacuumannealed, reduced TiO 2 surface itself produces ethylene as the main desorption product. Reoxidation of the TiO 2 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 Å diameter, 12.7 ( 2.4 Å height) is minimal, but trace amounts of formaldehyde, CO 2 , methane and H 2 are detected in TPD experiments, demonstrating that the Cu nanoclusters are more active than bulk single-crystal Cu surfaces. On oxygencovered Cu nanoclusters, methanol reaction produces formaldehyde and CO 2 as the major gaseous products as well as H 2 , water, and methane. The yields of formaldehyde and CO 2 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 18 O 2 prior Cu deposition results in the evolution of H 2 18 O, C 16 O 18 O, and C 18 O 2 in TPD experiments. No cluster size effects were observed for methanol chemistry on the pure Cu and oxygencovered Cu clusters.