It is well known that the transition from an insulator to a metallic conductor may be induced in amorphous semiconductor: metal alloys by increasing the metal concentration above a certain critical limit. However, without a detailed understanding of the changes taking place in the atomic scale structure, it is difficult to ascribe a mechanism to the process. We have investigated the microstructure of one such alloy system, a-Si 1 −yNi yH, using EXAFS as the principal technique. Thin film samples, prepared by rf co-sputtering, were studied over the composition range 0 < y < 0.3. Both silicon and nickel K-edge EXAFS results are presented, together with complementary data from Raman scattering, neutron diffraction and scanning calorimetry experiments. The results indicate that the samples contain two separate amorphous phases: a Ni:Si alloy which is embedded in the surviving, modified a-Si host network. The measured electrical conductivity is discussed in the light of this structural model.