Volume ionization produced in Ar by uv photoions was measured using a fast rotating Langmuir probe. The uv radiation was obtained from an electric discharge and collimated to produce a cylindrical plasma. Measurements were performed for pressures of 100, 200, 400, and 600 Torr and distances of 2.5–10.2 cm from the discharge. Plasma decay was observed from 50 to 350 &mgr;s after the discharge was fired and was attributed to radial expansion by ambipolar diffusion and volume recombination. By an extrapolation process, the initial photoion density can be obtained; its distance dependence suggests two‐photon ionization of Ar. The results are also compared to other measurements found in the literature.