In this study, the formation, crystal structure, optical and thermoelectric properties of ultrathin (UT) films of iron and chromium monosilicides are considered, which exhibit optical and thermoelectric properties characteristic of semimetals with a low density of states near the Fermi level and the main contribution of holes to the Seebeck coefficient in the temperature range 120–400 K and the transition to its negative values at T > 400 K. The power factor for FeSi and CrSi UT films versus temperature was calculated and ab initio calculations of the phonon structure and thermal conductivity for bulk FeSi and its nanowires were carried out, which made it possible to estimate the thermoelectric figure of merit of ultrathin FeSi films.