We have directly measured the birefringence&Dgr;nofCdGeAs2using polarized light interference spectra obtained in transmittance from 2.4 to 18 &mgr;m over a temperature range of 14–450 K. Four different samples, exhibiting a wide range of free carrier concentrations, were studied. It was found that free carriers can have a significant effect on the room-temperature birefringence. At 14 K, however, the data from all samples were virtually identical. The temperature dependence of&Dgr;nwas obtained from a sample with low carrier concentration(≈1015/cm3)and high resistivity (9 &OHgr; cm). Its temperature derivatived(&Dgr;n)/dTwas determined at 50 K increments over the entire 14–450 K temperature range, and was found to vary from2.6×10−5/Kat 100 K to5.1×10−5/Kat 400 K. A least-squares fit to the data yielded a temperature coefficient for&Dgr;nof8.2×10−8/K. Hall-effect measurements indicate that the resistivity, mobility, and carrier concentration all exhibit a significant temperature dependence. The resistivity for one sample increased by a factor of108when cooled from room temperature to 14 K. Our results suggest that previously published&Dgr;nvalues were probably obtained from samples with relatively high carrier concentrations, and that our results more accurately describe the true intrinsic birefringence of the material. The value of&Dgr;ndecreases by 0.0004 per1016carriers a factor of 10 less than suggested by theory but still significantly large. The heavy to light hole effective mass ratio in the uppermost valence band was found to be 7.1.