Abstract <p>The results of the study of the elemental and charge composition of the plasma of a vacuum arc discharge with a cathode made of magnesium Mg are presented. Time-of-flight spectrometry was used to determine the mass-charge composition of the ion current extracted from the vacuum arc plasma. The spectral characteristics of the discharge plasma were studied by recording the power density distribution of its radiation in the optical spectral range. Both methods were used simultaneously, so that a high level of identity was maintained not only in the operating conditions of the vacuum arc, but also in the true composition of the plasma of the cathode spots, which are the elementary cells of the structure of such a discharge. It was found that the fractions of heavy charged particles, represented by single- (Mg⁺) and double-charged (Mg²⁺) magnesium ions, in the beam extracted from the expanding plasma, depend significantly on the average energy deposited in the discharge. With the increase of the average energy the fraction of Mg²⁺ decreased, while that of Mg⁺ increased. In turn, the relative amplitudes of the lines of ions (Mg II and Mg III) and atoms (Mg I) of magnesium in the emission spectra of the plasma of cathode spots did not depend on the discharge energy characteristics. Based on the obtained results, reasoning is given about the mutual complementarity of the methods used and the further development of methods for diagnostics of cathode arc plasma.</p>