In order to determine amphetamine and metamphetamine, we have used a method never used before in the legal practice. The aim of our work was to optimize the measurement conditions for the rapid and accurate determination of amphetamine and metamphetamine. We have developed a method which makes possible the unambiguous identification of the hydrochlorides of the hallucinogens amphetamine and metamphetamine, even when these two are present simultaneously. In our work we have used the ammonium salt technique. The information available from the mass spectra (molecular ion, appearance of characteristic fragments) are dependent on the temperature of the direct sample and the ionisation energy. As a consequence, we have primarily studied the influence of the above two parameters upon the mass spectra.