Azimuthally symmetric ultraviolet (UV) preionization of the outer periphery of a gas puff z pinch, prior to current initiation, may reduce the growth of magnetically driven Rayleigh–Taylor instabilities or other nonuniformities affecting the final implosion stage of the pinch, leading to an improvement in K-shell x-ray yield. We report on measurements of a flashover UV photoionization scheme, capable of ionizing the periphery of argon gas puffs to 1%–10% of the initial gas density. Measurements are made with a two-color, (1064 and 532 nm), high sensitivity (∼10−5λ), Mach–Zehnder type interferometer. Two methods of measuring preionization are investigated. The first uses a single laser wavelength, 1064 nm, to probe a chord of the cylindrical gas puff. The gas density is measured first without preionization and then with the UV flashover discharge. The difference in phase shift determines the free electron density. The second technique uses both wavelengths to simultaneously probe the same line of sight, determining neutral and free electron densities using only one gas puff event. Gas preionization may introduce refractive species that complicate the conversion of phase shift to density. However, single-color interferometry with the IR and green beams indicates that it is sufficient to consider only two species, neural argon and free electrons for times of interest.