The process of vacuum gap formation in a microsecond plasma opening switch has been studied using a He–Ne laser interferometer. Time and spatially resolved density measurements are performed in the plasma opening switch, showing a density decrease during switch opening in almost the whole interelectrode gap. Despite fine triggering accuracy and generally good shot-to-shot reproducibility of voltage and current waveforms, measurements made with the same laser beam location indicated remarkable shot-to-shot variation. The unstable formation of the vacuum gap in the microsecond plasma opening switch may be a major limiting factor for efficient switch operation. ©1997 American Institute of Physics.