We are interested in understanding how metal nanoparticle size affects surface chemistry so that specific particle sizes with the desired reactivity can be identified for catalysis applications. Cu nanoparticles were grown on a TiO@sub 2@(110)-(1x2) surface and characterized by scanning tunneling microscopy under ultrahigh vacuum conditions. The Cu nanoparticles deposited on TiO@sub2@(110)-(1x2) exhibit the same "self-limiting" growth behavior previously observed on the unreconstructed titania surface: the particle density increases with increasing coverage while particle size is relatively constant. At all coverages, the Cu particles have a uniform size distribution, and the particle size can be controlled by annealing the surface to higher temperatures. Deposition at room temperature produces particles that are ~25 Å in diameter and ~5 Å high, while annealing to 700 K increases the particle size to an average diameter of 60-70 Å and height of 15-20 Å. We found that a smaller size regime of Cu nanoparticles can be prepared by depositing on this highly reduced titania surface compared to the more stoichiometric titania (1x1) surface. X-ray photoelectron studies of the thermal chemistry of dimethyl methylphosphonate (DMMP) on the smallest Cu nanoparticles (25 Å diameter) show that DMMP decomposition occurs below room temperature. Specifically, P-OCH@sub 3@ bond scission is nearly complete at room temperature, but all P-CH@sub 3@ bonds are not broken until much higher temperatures (550 K). Both phosphorous and carbon can be removed from the surface by heating to 800 K. Although studies of DMMP reaction on the TiO@sub 2@(110)-(1x2) surface show that decomposition of DMMP on titania itself commences around room temperature, our data also suggest that P-OCH@sub 3@ bond scission occurs more readily on the Cu nanoparticles.