In situ electrical resistivity measurement and structural analysis methods including MeV 4He+ ion backscattering, x-ray diffraction, Auger electron spectroscopy combined with Ar+ sputtering, scanning and transmission electron microscopies have been used to investigate the properties and characteristics of the Ta-Si thin alloy film system as a function of temperature. Stoichiometric TaSi2 films were deposited by double electron-gun coevaporation on Si, oxidized Si, and sapphire substrates. A distinct drop in resistivity near 300 °C has been determined to be a transformation of the amorphous to crystalline phase. The kinetics of the transformation has been obtained by isothermal treatment over the temperature interval of 240-280 °C. The results are interpreted in terms of a classical solid-state phase transformation model with a t4 (time) dependence and an apparent activation energy of 1.85 eV. Subsequent annealing causes further decrease of resistivity. The microstructures of films at various stages of annealing have been studied by x-ray diffraction and transmission electron microscopy. Correlation between the resistivity and microstructure is given and discussed. In situ resistivity of annealed films below room temperature has been measured. Crystalline TaSi2 thin films do not become superconductive at 1.5 °K. Ion-beam mixed, coevaporated TaSi2 film and films varying in composition from TaSi3.4 to TaSi1.2 prepared by cosputtering have also been studied. The results are compared and discussed.