The temperature dependence of the conductivity (between 15 and 300 K) and optical transmission spectra (between 0.8-3.5 mu m) have been measured on r.f.-sputtered Si100-xNix with 0 less than or equal to x less than or equal to 15. The films were characterized by Rutherford backscattering spectroscopy and X-ray diffraction. Changes in the optical sub-bandgap structure, with corresponding changes in the conduction mechanism take place by varying the Ni content. The films are amorphous, showing a main broad diffraction peak. Its position deviates, for this concentrationrange, from 2(kF) (k(F) = Fermi radius), the alloy losing the Hume-Rothery character present for 50 less than or equal to x less than or equal to 60.