A study of the electrical resisitivity of (Fex.Mn1−x)Si with 0 < x < 1 indicates a crossover from an almost semiconducting state in FeSi to a metallic one in MnSi. Optical measurements show that the electronic structure and the vibrational behaviour appears to be essentially uneffected by the Fe/Mn substitution. Instead, a change of the free carrier density occurs, yielding a temperature and stoichiometry dependent plasma frequency.