Modulated elemental reactants containing iron and antimony were found to react at low temperature (T < 200 C) forming either FeSb2 or FeSb3 depending on both the layer thicknesses and the overall composition of the initial reactant. For films containing 75% antimony and 25% iron, the metastable compound FeSb3 was observed to nucleate and grow if the layer thickness was below approximately 35 Angstrom. Above this critical thickness for the modulation, annealing led to the formation of FeSb2. This, combined with low-angle diffraction data, suggests that the initial interdiffusion between iron and antimony layers in an elementally modulated reactant results in the formation of an amorphous reaction intermediate if the layering thickness is less than 35 Angstrom. For modulated reactants with composition between 70 and 90 atomic % antimony and below this critical layer thickness, the metastable compound FeSb3 formed. In more iron-rich modulated reactants FeSb2 nucleates. The nucleation temperature and the nucleation energy of FeSb3 were found to be a function of the composition of the amorphous intermediate, while those of FeSb2 were found to be relatively independent of composition.