Formaldehyde molecules in interstellar dust clouds condense on interstellar silicate grains as polyoxymethylene whiskers. A significant fraction of interstellar O and C atoms may be frozen onto grains in this form, and this material could be responsible for a major part of the observed extinction and polarization of starlight at optical wavelengths. Polyoxymethylene grains have a spectral band in the wavelength region from 8–12 µ, similar to that observed in stellar spectra.