Cs-chabazite, a type of zeolite with caesium counter-cations, possesses interesting gas separation properties due to a highly selective molecular “trapdoor” effect. Herein the use of this material for H 2 /D 2 isotope separation is demonstrated. Isotope separation was achieved using breakthrough separation with a single pass through a packed bed at moderate temperatures (293 K) and pressures (0.17 MPa) when one species was in a sufficiently low concentration. The breakthrough separation curves were successfully modelled using the Thomas kinetic breakthrough model and the Yoon and Nelson kinetic breakthrough model, where working transferable kinetic rate constants were developed. Use of this material for hydrogen isotope separation would significantly lower the total energy demand compared with current hydrogen isotope separation techniques such as cryogenic distillation and is applicable to separating out low concentrations of D 2 (0.0156%) present in standard grade H 2 .