Observations of line-emission Doppler broadening and Doppler shift are used to determine the velocity distributions, parallel and perpendicular to the anode surface, of neutral particles and multiply charged ions in a surface-flashover-produced plasma in a magnetically insulated ion diode. The velocity distributions were found to be nearly Maxwellian with thermal energies of 8 eV for neutral particles, 20 eV for singly charged ions, and 20-80 eV for doubly and triply charged ions. The plasma-pressure gradient in the magnetic field, obtained from the observed ion temperature and density, and a suggested anomalous plasma conductivity that is approximately ten times lower than the classical conductivity are shown to explain the fast plasma expansion with respect to the magnetic field. Elastic collisions of the 7-eV electrons with the hotter ions cause electron heating. The observed ion velocities in the plasma parallel to the anode account for (1/3 of the velocities in the same directions observed in the diode acceleration gap in a previous experiment.