The coordination and spin equilibrium of a Ni-II meso-tetra(4-carboxyphenyl)porphyrin compound, NiP, was quantified both in fluid solution and when anchored to mesoporous, nanocrystalline TiO2 thin films. This comparison provides insights into the relative rate constants for excited-state injection and ligand field population. In the presence of pyridine, the spectroscopic data were consistent with the presence of equilibrium concentrations of a 4-coordinate low-spin S = 0 ((1)A(lg)) Ni-II compound and a high-spin S = 1 (B-3(lg)) 6-coordinate compound. Temperature-dependent equilibrium constants were consistently smaller for the surface-anchored NiP/TiO2, as were the absolute values of Delta H and Delta S. In the presence of diethylamine (DEA), the ground-state 6-coordinate compound was absent, but evidence for it was present after pulsed light excitation of NiP. Arrhenius analysis of data, measured from -40 to -10 degrees C, revealed activation energies for ligand dissociation that were the same for the compound in fluid solution and anchored to TiO2, E-a = 6.6 kcal/mol, within experimental error. At higher temperatures, a significantly smaller activation energy of 3.5 kcal/mol was found for NiP(DEA)(2)/TiO2. A model is proposed wherein the TiO2 surface sterically hinders ligand coordination to NiP. The lack of excited-state electron transfer from (NiP)-P-II*/TiO2 indicates that internal conversion to ligand field states was at least 10 times greater than that of excited-state injection into TiO2.