A method has been developed for determining photometrically the ionization of multiplate cloud chamber tracks. A simple, projection type of photometer is used to measure the track opacity, from which the ionization of the track is obtained by comparison with the opacity of a track of known ionization. The ratio of the properly normalized opacities of the two tracks is related to the ratio of their ionizations by a simple power law. The technique has been employed, with geometric residual range determinations, to find particle masses. It also has been used, when the mass is known, to ascertain the residual range in the plate in which a particle stops. By determining both the ionization and the change in ionization occurring in a plate traversal, both the mass and the residual range of particles have been found. Specific applications include a determination of the range of the secondaries fromK&mgr;2andK&pgr;2decays, and a measurement of the mass of a particle which releases more than 1.6 Bev on stopping. The latter is found to have a mass consistent with its being an antiproton.