Abstract: A equipment for the measurements of photo-conductivity and photo-Hall effect, which provides a useful tool to investigate the influences of deep level impurities on the properties of semi-insulating materials, has been developed. The results at RT and 77 K by using this equipment on several different types of SI-GaAg crystals are reported. We also discuss some features of spectra obtained from these measurements. in view of the existence of Cr and O levels in crystals.