The principles and experimental results of a frequency spectrum analyzer for all types of single transient processes are described in this paper. By the suitable combination of existing laboratory instruments and a few special circuits which are described here, the analyzer may be assembled fairly quickly. By using time expansion (or compression) techniques, the frequency spectrum of interest may be trans-fered to any frequency band. The single transient is reproduced as a repetitive waveform by means of a photoformer. In this way it is possible to use an ordinary low frequency spectrum analyzer to measure the frequency spectrum of single transient processes of any type or duration. Experimental results prove that the above method is applicable. For the principal compositions of the spectrum, measurements may attain an accuracy of 2-3%. The accuracy for minor compositions of the spectrum is somewhat less.