The crystalline quality of GaAs and InP single crystals grown by the liquid-encapsulated Czochralski technique, was investigated by gamma-ray diffraction. In particular, the rocking curve was measured in different areas of each crystal, obtaining the mosaic spread, which characterises the grain disorientation around the average direction. The analytical technique which was used, proved useful for the non-destructive characterisation of nearly perfect crystals. In particular, the crystalline quality of Si-doped GaAs and S-doped InP single crystals was found to be very high, whereas the undoped InP crystal appeared to be of worst quality, with a mosaic of the order of few minutes and varying from place to place.