The decomposition of deuterated formic acid (HCOOD) on a partially oxidized nickel (110) surface was examined by flash desorption techniques. The clean surface was oxidized by exposure to oxygen at room temperature in amounts varying up to 10 L. Auger electron spectroscopy (AES) and low energy electron diffraction (LLED) were used to characterize the surface. For oxygen exposures of less than 20 L the autocatalytic mechanism producing D 2 and CO 2 previously observed on the clean surface was preserved, and a second mechanism, characterized by a broad peak including one or more simple first-order processes, was introduced. As oxygen exposure was increased, the explosive peak was displaced to higher temperatures and its magnitude diminished while the broad peak increased in size. On the ordered one-half monolayer oxygen surface the rate constant for the autocatalytic process decreased from the clean surface value of 1.6 × 10 15 exp(−26600/RT) sec −1 to 4.0 × 10 11 exp(−20800/RT) sec −1. On the same surface, the rate constant for the broad peak was 1.6 × 10 12 exp(−21200/RT) sec −1. The amount of CO evolved equaled the amount of CO 2 produced by the autocatalytic process, indicating the importance of an anhydride intermediate. Two intermediates were thus involved in the production of CO 2 on the oxygen chemilayers; the first was an anhydride, as on the clean surface, and the second was a HCOO species. The presence of oxygen on the Ni(110) surface thus exhibited two effects. Preservation of the autocatalytic mechanism with an alteration of the rate constant indicated a comparatively mild perturbation of the nickel electronic environment, while the introduction of a second mechanism, which became dominant was oxygen exposure was increased, was characteristic of a much stronger oxygen-nickel interaction.