[en] The initial energy spread of ions formed in vacuum spark-discharge plasma is studied, and is found to be approx. 100 eV (at 50% of maximum intensity). Of three possible mechanisms for ion acceleration, the hydrodynamic mechanism and the action of the self-consistent electric field have been shown to be the basic mechanisms. The mechanism of ion acceleration in the inter-electrode field does not play an important role. The energy spread of ions can increase considerably during the formation of an ion beam, and this is usually the case in double-focusing mass spectrometers. (orig.)