Optical transmittance measurements in the temperature range of 80-380 K have been carried out on semiconducting p-iron disilicide layers formed by annealing of iron firms evaporated onto silicon substrates and capped with amorphous silicon thin overlayers. The dependence of the absorption coefficient on the energy of photons revealed direct allowed transitions. The application of a simple three-parameter semiempirical formula to the temperature dependence of the direct energy gaps leads to the following best fit parameters : the bandgap at zero temperature E-g(0) =0.894+/-0.0066 eV, the dimensionless coupling parameter S=1.98+/-0.31, and the average phonon energy ((h) over bar omega) = 46.4 +/- 9.8 meV (averaging for three samples). Comparison of these parameters also with those reported for beta-FeSi2 indicates that, although the bandgaps at zero temperature are very similar, both the coupling parameters and the average phonon energy may vary with samples fabricated by different techniques and thermal processes.