The results of resonance absorption measurements in cesium vapor are reported. It is shown that the half‐height width of the absorption (&ggr;) is directly proportional to the neutral cesium density. Over the reduced pressure range of 0.2 to 10 Torr, &ggr;k=10−22cm3, wherekis the photon‐atom absorption cross section. Ionization measurements are reported describing the effect of the absorption of resonance radiation in cesium vapor. On the basis of the results presented, an ionization mechanism has been hypothesized for the production of the cesium molecular ion (Cs2+) as the dominant ionic species in high‐pressure, low‐energy cesium vapor thermionic plasmas. The mechanism is based on the collision of two cesium atoms excited to the resonance level (1.45 or 1.38 eV). The excitation mechanism shifts from nearly complete photon‐induced excitation at a cathode temperature of 2000°K to equal contributions of photon induced and electron induced excitation at a cathode temperature of 1550°K. At lower cathode temperatures, the excitation is accomplished primarily by electron impact processes. Single impact electron ionization processes are shown to be inadequate for neutralization in the plasma of low‐energy discharges such as those found in cesium vapor thermionic converters. It is shown that for a condition of space charge neutralization in the plasma of low‐energy cesium vapor discharges the ratio of excited state population density to ion population should be between 2.5 and 5 to 1.