Plasma opening switches have been studied by using laser-produced plasmas. In this paper, a CO/sub 2/ laser (5 J/pulse, 5.5 MW peak power and 10.6 /spl mu/m wavelength) was used as a plasma source of the plasma opening switch (POS). The plasma parameters were measured by Faraday cup, Langmuir probe and a monochromator. It was found that ions with a fast flow velocity are required to obtain the fast opening time, and thermalized plasmas with a slow velocity decrease the opening time. However, the POS operation was improved by the switch modifications despite the existence of thermalized plasmas.