Electrical and microstructural changes of NiSi2±0.2 alloy thin films coevaporated on two different substrates were studied as a function of temperature. In-situ resistivity measurements, transmission electron microscopy, and X-ray diffraction were applied. The substrates used were: a) silicon single crystal grown in the [100] direction, and b) similarly grown single crystal but thermally oxidized to a depth of 500 nm. Substantial differences were observed in the morphology and the kinetics of crystallization on both substrates. It was found that the crystallization temperature depends on the composition of the deposited film as well as on the type of substrate. An increase in the Si content of the alloy film deposited on the inert substrate lowers the crystallization temperature, while with the active substrate a rise in the crystallization temperature was observed. The final crystallized phase was identified as NiSia for the alloy films codeposited on Si. For the alloy film deposited on the thermally oxidized substrate NiSi and NiSia were observed when Ni content was higher than 33.3 at.% The crystallization process occurs by homogeneous growth in the film on either substrate. Heating to higher temperature probably results in regrowth of epitaxial MiSis accompanied by the appearance of stacking faults in the alloys deposited on Si (100).