The Hall mobility in semi‐insulating GaAs has been measured by probing the Hall voltage dropped across the thickness of the sample optically. This is achieved by exploiting the linear electro‐optic (Pockels) effect in the material. The method only involves two electrical contacts to the sample which gives a distinct advantage over the conventional Hall or Van der Pauw techniques. It permits, in principle, the direct measurement of the time evolution of carrier populations, a useful corollary to transient photoconductivity. It also makes possible the measurement of a Hall voltage dropped across the width of a quantum well.