Abstract A possible astrophysical formation process of FeO and its solid solution with MgO, called mag-nesiovvüstite (Fe x Mg 1 − x ,O), is the condensation in the oxygen-rich envelopes around M stars. This process, in addition to silicate formation, is only expected under special oxygen partial pressure and on temperature conditions in an Si-poor or Fe-rich environment. For the sake of comparison with IR observations, the refractive index of laboratory-prepared magnesiowüstite samples (Fe x Mg 1 − x ,O, x = 1.0. 0.9, 0.8., 0.7., 0.5, 0.4) in the wavelength range from 10 to 500 μm were calculated from reflectance spectra. For small particles in the Rayleigh limit (radius ⪡ λ), the peak wavelengths of the absorption coefficient range between 20 (FeO) and 18 μm (Fe 0.4Mg 0.6O).