We have measured the birefringence(&Dgr;n)of ZnGeP2throughout the 14–500 K temperature range using polarized light interference spectra obtained in transmittance from 0.7 to 12 &mgr;m. It was found that&Dgr;ncontinuously increases with temperature for all wavelengths but that the amount of increase is not constant with temperature as often assumed.&Dgr;nincreases by approximately 0.006 when the temperature changes from 14 to 500 K. The temperature derivative of the birefringence,d(&Dgr;n)/dT, was determined at 50 K increments and its value was found to be quite dependent on the exact temperature range over which it was measured. It increases linearly with temperature, having a coefficient of 3.3×10−8/K. Actuald(&Dgr;n)/dTvalues range from 5×10−6/K at the lowest temperature to 19×10−6/K at the highest. We conclude that the anomalously low phase matching angles observed for second-harmonic generation in ZnGeP2are not associated with errors in birefringence values as previously speculated but instead are most likely due to nonlinear absorption phenomena.