Positron annihilation (PA) is a sensitive technique for detection of vacancy-type defects in crystals, that has been widely used in recent years to study defects in semiconductors (1). On the other side, CL and other luminescence techniques have been applied (2) to study the defect distribution in semiconductor wafers. In some cases PA can be useful to interpret results obtained by CL-SEM (3). In this work PA and CL have been used to investigate the distribution and nature of defects in GaP : S, GaAs : Te and undoped SI GaAs wafers. CL intensity, dislocation density and vacancy concentration profiles have been measured. The latter has been obtained by positron lifetime measurements.