We measured the ellipsometric angles as a function of photon energy from 0.76 to 6.6 eV of 10 nm thick Ni1 - xPtx alloy (0 < x < 0.25) films deposited on thick thermal oxides. Using basis spline functions and Drude-Lorentz oscillator fitting, we determined the dielectric functions and optical conductivities of our alloy films. We describe techniques to increase the accuracy of our measurements and data analysis. We find absorption peaks near 1.6 and 4.8 eV due to interband optical transitions. There is a significant broadening of these peaks with increasing Pt content. Annealing the metals at 500 degrees C for 30 s increases the optical conductivity. (C) 2013 Elsevier B.V. All rights reserved.