The Chapman‐Enskog solution of the Boltzmann equation is used to obtain theoretical data for the viscosity of gaseous cesium, including the effects of up to 10% ionization. Results are presented for the viscosity of ionized cesium at pressures of 10−2, 10−3, 10−4, and 10−5atm in the temperature range 1000°–3500°K, and for unionized cesium over the same range. The viscosity of the 10% ionized gas is found to be about 20% lower than that of unionized cesium under the same conditions.