Explosions of 0.03 - 0.25 mm diameter wires of Cd, Al, Cu, and W in high vacuum (p < 10-5 Torr) are investigated. The time development of the discharge column is shown to be determined by two main processes: a) In the time preceding the explosion, particles are emitted from the surface of the heated wire and initiate a peripheral discharge. The mechanism of particle emission is found to be evaporation of neutral atoms and/or thermionic emission of charged particles. The latter process is influenced by the strong radial electric field which is caused by the coaxial discharge geometry at the wire surface, b) The wire material vaporizes explosively forming an expanding cloud of nonconducting vapor which is subsequently converted to a plasma by the peripheral discharge penetrating into it from outside. The discharge column exhibits instabilities which are shown to be of MHD origin. They are significantly reduced by applying an axial magnetic field to the discharge column. A quantitative spectroscopic investigation is performed during the magnetic contraction phase of stabilized 0.05 mm Al wire explosions. The plasma temperature is found to be about 50 000 °K in the axis of the column and higher than 80 000 °K at its periphery. The mean electron density is estimated to be of the order of some 1019 cm-3. © 1969, Walter de Gruyter. All rights reserved.