In the present paper, discussion has been focused upon the problem of identifying toxic substances with the aid of the GC/MS system, i.e. gas chromatography - mass spectrometry. Factors involved in a successful combination of a gas chromatography with a mass spectrophotometer have been pointed out. In addition attention has been drawn to the two various ways of obtaining the spectra, i.e. the classic EI spectra and those obtained during chemical ionization, the mode of performing mass fragmentography and, last, the importance and characteristic features of the computer-controlled GC/MS systems.