A method and an apparatus used for measuring of the deep level content of semiconducting crystals are described. The investigation is based on the observation of capacitance (voltage) relaxation of diodes. The evaluation of the relaxation curve is the same as proposed first byLang [1], the so-called DLTS (deep level transient spectroscopy) method. The difference between the apparatus presented here andLang’s original is the using of a commercial capacitance meter and the possibility of the measurement of voltage relaxation at fixed capacitance value. Deep centres in GaAs or GaP layers having ionization energies between 0,2… 1 eV can be detected if their concentration is not less then 3·10−3 part of the shallow donor (acceptor) concentration.