The adsorption of zirconium nitrate [Zr(NO3)4, ZN] on thin films of polycrystalline zirconia (ZrO2) was investigated with temperature-programmed reaction mass spectrometry (TPR) and X-ray photoelectron spectroscopy (XPS). TPR measurements demonstrate that adsorbed ZN undergoes competitive desorption and reaction near 340 K. The reaction is the first in a series of steps that ultimately leads to ZrO2 formation. Several gas-phase products are formed as a result of ZN decomposition, including NO, NO3, and O2. The gas-phase products evolve from two temperature envelopes centered at ∼350 and ∼400 K, with the O2 envelopes at somewhat higher temperature than the NO and NO3 envelopes. Partitioning between the desorption and decomposition pathways is a function of the heating rate, β, during TPD, with the ZN desorption fraction varying between 0.05 at β = 1 K s-1 and 0.4 at β = 8 K s-1. The XPS measurements reveal that the zirconium oxidation state is constant at +4 during the decomposition sequence. However, the identities of the gas-phase products suggest that the NO3 ligands undergo oxidation-reduction chemistry during decomposition. On the basis of the TPR and XPS measurements, it is proposed that adsorbed peroxides of formula Zr(NO3)2(O2) and ZrO(O2) are intermediates on the decomposition pathway. Both peroxides contain states of oxygen that are oxidized relative to oxygen in ZN.