The Pt_1−xNi_x solid solution has been investigated using density functional theory (DFT) calculations. Pt-based bimetallic alloys are currently used as alternative bifunctional electrode materials for the electro-catalytic oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) underpinning the technology required for regenerative fuel cells. The work involved studying the mixing thermodynamics and electronic structure of the solid solution with Pt and Ni as end members. The data described here are ASCII files containing the data for the configurational entropy and the mixing enthalpy as function of composition; the probability distribution of energies for the equilibrium composition at room temperature and for the fully disordered system; the density of states for the end members of the solid solution; as well as the electronic band structure along the atomic charges and magnetic moments for the two major configurations of the equilibrium composition. Calculations were carried out using the Vienna Ab-initio Simulation Package (VASP).