The mechanism of Ni substitution into the oxide semiconductor InTaO4 has been studied through a combination of structural and spectroscopic techniques, providing insights into its previously reported photoactivity. Magnetic susceptibility and X-ray absorption near-edge spectroscopy (XANES) measurements demonstrate that nickel is divalent within the host lattice. The combined refinement of synchrotron X-ray and neutron powder diffraction data indicates that the product of Ni doping has the stoichiometry of (In1-xNi2x/3Tax/3)TaO4 with a solubility limit of x approximate to 0.18, corresponding to 12% Ni on the In site. Single-phase samples were only obtained at synthesis temperatures of 1150 degrees C or higher due to the sluggish reaction mechanism that is hypothesized to result from small free energy differences between (In1-xNi2x/3Tax/3)TaO4 compounds with different x values. Undoped InTaO4 is shown to have an indirect band gap of 3.96 eV, with direct optical transitions becoming allowed at photon energies in excess of 5.1 eV. Very small band-gap reductions (less than 0.2 eV) result from Ni doping, and the origin of the yellow color of (In1-xNi2x/3Tax/3)TaO4 compounds instead results from a weak (3)A(2g) -> T-3(1g) internal d -> d transition not associated with the conduction or valence band that is common to oxide compounds with Ni2+ in an octahedral environment.