A classical microscopic theory is presented for vibrational excitons interacting with the electromagnetic radiation field in uniaxial and biaxial molecular crystals. The coupled exciton—photon field leads to an eigenvalue equation, the solutions of which are polariton modes. It is shown how their frequencies ω(k) can be calculated, if the oscillator strengths are known. It is further shown, that there is a close connection between factor-group splitting and LO/TO splitting. That part of the effective field, whose range is bounded by the surface of the Lorentz sphere, produces factor-group splitting, while LO/TO splitting is caused by the long-range part of the effective field, which is produced by the matter outside the Lorentz sphere.