The uniaxial stress‐induced coupling of the optical ordinary and extraordinary modes in CdS is quantitatively investigated by analyzing, at room temperature, the transmitted light spectrum of a sample placed between two polarizers crossed along the natural neutral lines. Experiments are performed in the wavelength range 5200–7000 A˚ under application of a calibrated (<500 bar) uniaxial stress directed at 45° with respect to the C axis. At the near gap isotropic wavelength 5229 A˚ the phase matching allows energy transfer. It is shown that the high selectivity of the process is due to the fast rotation of the induced neutral lines. Due to the stress‐induced splitting, the actual birefringence exhibits a sharp but nonzero minimum, at the isotropic wavelength, which is a linear increasing function of the pressure. Its linear pressure coefficient is measured, 6.3×10−6bar−1, and this allows a direct calculation of the corresponding elasto‐optic coefficientp44= 9.9×10−2. The frequency dispersion ofp44is investigated near the fundamental absorption edge where it exhibits a broad maximum and a sharp structure. The main characteristics of a coupled mode band‐pass optical filter are calculated and briefly discussed. Finally the residual internal strain field in most ’’as‐grown’’ crystals is estimated as 1.5×10−5.