The photoconductive (PC) and photoelectromagnetic (PEM) effects have been measured for a number of specimens of semi-insulating gallium arsenide (resistivity greater than 10 5 Ω cm ) . The results enable both majority and minority carrier lifetimes to be determined and some values are given. In the small signal region, the lifetimes are found to be only slightly dependent on intensity of illumination, indicating an essentially linear relationship between intensity of illumination and photo-signal. For specimens at room temperature, the two lifetimes are of the same order of magnitude, which shows that trapping effects are small. A check on these measurements can be obtained in principle from a measurement of photoconductive decay time constants. The photoconductive spectral response shows a tail on the long wavelength side of the absorption edge extending to approximately 2 μ. This has been ascribed to a level lying deep in the band gap with a threshold energy for photoconductivity of 0.63 eV.