The thermal decomposition of dimethyl methylphosphonate (DMMP) has been studied in ultrahigh vacuum by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) on Ni clusters and films deposited on TiO2(110). The four different Ni surfaces under investigation consisted of small Ni clusters (5.0 +/- 0.8 nm diameter, 0.9 +/- 0.2 nm height) deposited at room temperature and quickly heated to 550 K, large Ni clusters (8.8 +/- 1.4 nm diameter, 2.3 +/- 0.5 nm height) prepared by annealing to 850 K, a 50 monolayer Ni film deposited at room temperature, and a 50 monolayer Ni film annealed to 850 K. The morphologies of the Ni surfaces were characterized by scanning tunneling microscopy (STM). TPD experiments show that CO and H-2 are the major gaseous products evolved from the decomposition of DMMP on all of the Ni surfaces, and molecular DMMP and methane desorption were also observed. The product yields for CO and H-2 were highest for reactions on the small Ni clusters and unannealed Ni film and lowest for reactions on the large clusters and annealed film. Furthermore, XPS experiments demonstrate that the unannealed Ni surfaces decompose a greater fraction of DMMP at room temperature. The loss of activity for the annealed surfaces is not caused by a reduction in surface area because the annealed surfaces have approximately the same surface area as the small clusters. CO adsorption studies suggest that the loss of activity upon annealing cannot be completely due to a decrease in surface defects, such as step and edge sites, and we propose that a TiOx moiety is responsible for blocking active sites on the annealed Ni surfaces. In comparison to the TiO2 surface, the small Ni clusters are more chemically active because a greater fraction of DMMP decomposes at room temperature, and the total amount of DMMP decomposition is also higher on the small Ni clusters. Although DMMP decomposes on TiO2 to produce gaseous methyl radicals, methane, and H2, the activity of the substrate surface itself appears to be quenched in the presence of the Ni clusters and films. However, the TiO2 support plays a significant role in providing a source of oxygen for the recombination of atomic carbon on Ni to form CO, which desorbs above 800 K.