The environmental and toxicity concerns of lead in hybrid organic-inorganic perovskite (HOIP) solar may become a limiting factor in the large-scale and widespread deployment of these devices. Thus, there are continuing efforts to replace Pb 2+ with other ions in the crystalline structure while maintaining the optical and electronic properties. The most obvious choices to replace Pb 2+ are the group 14 ions such as Sn and Ge , which have been extensively studied. However, thus far, these ions have been met with limited success in part due to their instability to oxidation. Therefore, we need to need explore other alternatives. A number of transition metal ions are of particular interest in the replacement of Pb 2+ due to their rich oxidation chemistry, low toxicity, and relative abundance. In fact, computational screening studies predict that HOIPs containing metal ions such as Mg 2+ , V 2+ , Mn 2+ , and Ni 2+ are likely to function as direct bandgap semiconductors and are good candidates for replacing Pb 2+ ions. However these predictions have not been experimentally verified. Herein, we study the optical and structural properties of methylammonium lead triiodide (MAPbI3) films with varying degrees of Ni substitution. We show that the three-dimensional (3D) perovskite structure is maintained with increasing ratios of NiI2 of up to 30% Ni 2+ . Interestingly, we observed that the mixing of NiI2 and PbI2 precursors did not produce uniformly mixed films in the solid-state, as analyzed by Xray diffraction and visible absorption studies. We report optical bandgaps for the Ni 2+ substituted films between 1.3 and 1.6 eV, which appear to be independent of the Ni 2+ concentration. Furthermore, we observe quenching of emission from MAPbI3 upon Ni 2+ substitution, indicating the Ni 2+ may act as a recombination center for charge carriers. Lastly, the Ni 2+ substituted films displayed high sensitivity to water, as observed by rapid color changes of the films on exposure to ambient conditions. Thus, despite the optimal bandgap and structure of the materials, we conclude that Ni 2+ is not a promising candidate for Pb 2+ replacement in these conditions (e.g. polycrystalline thin-films deposited from NiI2 precursors) for HOIP solar cells.