Measurements are reported on semiinsulating p-type Gallium Arsenide specimens illuminated with photons of energy greater than the energy gap. An excited layer is defined of thickness d given by the sum of the radiation penetration depth and the ambipolar diffusion length. Concentrations n and mobilities μ of the carriers in this layer are determined from galvanomagnetic effects in light and in darkness. The concentration n shows a slow decrease with increasing wavelength out to the absorption edge, where it falls abruptly; μ falls with rising photon energy. As a function of increasing intensity, the mobility remains constant, while n rises linearly with photon-excitation rate. The results are discussed in terms of the variations of d. Further an attempt has been made to find the dependence between electron and hole concentrations in the excited layer measuring the variations of short circuit photomagnetoelectric current (IPME) as a function of excess conductance (ΔG).