A method for determining charged energy states of a sample of a semiconductor or insulator material by using deep level transient spectroscopy. The method includes the steps of exciting the sample by application of periodical exciting pulses to change the initial charge state, detecting the transient response of the sample when it returns to the thermodynamical balance condition following the termination of each of the excitation pulses, blocking the detection during a blocking period defined as the combined existence of the exciting pulses and of a dead period including the recovery period of the means used for the detection, performing a weighted integration operation on a detected response signal by the application of a symmetrical square wave synchronizing pulse as a weighting function synchronized to the frequency of said exciting pulses, blocking the detection in each of said detecting periods for the duration of a further blocking period which begins in a moment defined between the starting moments of the two exciting pulses immediately preceding and following said detecting period and synchronizing the synchronizing pulses to terminating moments of said dead periods to have a period time which is equal to the period time of said periodical exciting pulses. Apparatus for carrying out the method is also disclosed.