Thin film reactions between 100 nm of Ni and 400 nm of Ge deposited on thermally oxidized Si wafers have been studied as a function of annealing conditions, using X-ray diffraction (XRD), cross-sectional transmission electron microscopy, four-point kelvin resistance measurements and X-ray photoelectron spectroscopy (XPS); these last two techniques were carried out during reaction. The real-time resistance characteristic was nearly identical for temperature ramp rates between 3 and 50 degrees C min(-1) over the temperature range 20-550 degrees C, suggesting that the same basic reactions path dominates for all these conditions. The initial room temperature sheet resistance of the bilayer structure is about similar to 2 Omega/square and, upon annealing, goes through a local maximum of similar to 6 Omega/square at 300 degrees C. XPS indicates that Ge first appears on the sample surface, while XRD indicates the presence of monoclinic Ni5Ge3 at this sheet resistance maximum. Orthorhombic NiGe nucleates and grows between 300 and 400 degrees C. Analysis of resistance vs. time during isothermal annealings at 200-300 degrees C suggests the formation of NiGe through a thermally activated process with activation energy E(a) = 1.3 eV. At temperatures approaching 500 degrees C, XPS shows the disappearance of the Ni signal from the surface, indicating the onset of agglomeration.