The room‐temperature reststrahlen spectra of flame fusion grown NiO and CoO, as well as those of three intermediate members of this solid solution series, are presented. A full Kramers‐Kronig analysis on all members has been performed and checked with damped oscillator calculations. The resulting refractive indices, extinction coefficients, dielectric constants, effective ionic charges (Szigetiq*), and characteristic frequencies are presented along with the lattice constants and are discussed in the light of their interrelation.