Conductivity, ionic transference number, and chemical diffusion coefficients are determined for KO2, RbO2, and CsO2. Based on such results, a defect-chemical model is constructed. These superoxides are found to exhibit a total conductivity in the range of 3 x 10(-7) to 5 x 10(-5) S cm(-1) at 200 degrees C with contributions from ionic and electronic carriers. The ionic conductivity is caused by alkali interstitials and superoxide vacancies as mobile defects, and is found to exceed the n-type electronic conductivity. O-18 isotope exchange on powder samples (monitoring the gas phase composition) shows that essentially all oxygen can be exchanged. At high pO(2) this largely occurs without breaking of the O-O bond-indicating a sufficient mobility of molecular superoxide species in the solid-and with an effective rate constant that is much higher than for other large-bandgap mixed conducting materials such as SrTiO3.