Structural and optical measurements have been made on a series of amorphous thin films of Si_1-xNi_x with 0<x<0.75, prepared by RF sputtering. We present the results of structural studies using extended x-ray absorption fine structure (EXAFS) and small angle x-ray scattering (SAXS) together with optical measurements of the band gap. The EXAFS analysis gave interatomic distances, partial coordination numbers and Debye-Waller factors for the nearest neighbours of the two atom types. The Ni environment was found to be independent of composition for x<0.5. A small variation in the mean Si-Si distance was also found in this composition range. The SAXS results show phase segregation, on the 30 Å scale, for this composition range. We interpret the structural data in terms of a two-phase model, amorphous Si plus amorphous NiSi, for x<0.5 and a single-phase model for x>0.5. The optical data show that a metal-insulator transition occurs at about 10-15% Ni content. We suggest that this transition should be interpreted in terms of a percolation theory involving regions of conducting amorphous NiSi in an amorphous Si matrix.