The birefringence (&Dgr;n) of ZnGeP2has been measured directly from polarized light interference spectra obtained in transmittance over the 0.66–12 &mgr;m wavelength range from samples of six different thicknesses. The &Dgr;nvalues were determined from the positions of fringe maxima (&Dgr;n=k&lgr;/t) and then compared to previously published data which were obtained by a different technique. It was found that the interference fringe method results in values of &Dgr;naccurate to ±0.00005. The data are shown to exhibit much less scatter as a function of wavelength than previous results and can lead to more accurate calculations of phase‐matching angles for second‐harmonic generation applications. ©1995 American Institute of Physics.