The theory of a probe to measure local atom concentrations in hypersonic flows of a dissociated gas at low densities is presented. The probe operates on a principle of measuring the differential heat transfer between catalytic and noncatalytic gauges immediately adjacent to one another on the stagnation line of a cylindrical model. The theoretical performance of the probe in the free molecule, the viscous shock layer, and the boundary layer regimes, respectively, is considered. It is shown that the probe cannot be operated successfully in the free molecule regime, except under unusual circumstances. In continuum flow, it is shown that under those conditions necessary for the atoms in the free stream to reach the surface of the probe (chemically frozen shock layer and boundary layer), we are always working in the viscous shock layer regime. Accordingly, it is this flow regime in which the probe measurements can be interpreted readily. The results of detailed calculations which relate to the application of this probe to measure local atom concentrations in nonequilibrium, hypersonic nozzle flows are presented.