The physical processes of the acceleration gap of flashover-type magnetically insulated pulsed ion diodes were examined. Spectroscopically measured values of the density and temperature of the anode plasma were 10/sup 16/ cm/sup -3/ and 1 eV, respectively. The electron temperature increased with the diode time. The anode plasma expansion velocity was estimated to be about 3 cm/ mu s. Resonant laser interferometry was applied to the plasma using a tunable dye laser pumped by a nitrogen laser. The hydrogen atom number density was estimated to be 10/sup 16-17/ cm/sup -3/. It filled the entire A-K gap within 200 ns from the time the voltage pulse rising started. Results are presented on the formation and evolution process of the anode plasma and the behavior of neutral particles in the gap. >