In the development of HC traps for reducing vehicle cold start HC/NOx emissions, zeolite based adsorbant materials were studied as key components for capture and release of the main gasoline-type HC/NOx species in vehicle exhaust gas. Typical zeolite materials capture and release certain HC and NOx species at low temperatures (<200°C), which is lower than the light-off temperature of a typical TWC catalyst (≥250°C). Therefore, a zeolite alone as an HC/NOx storage component in the HC/NOx trap is not effective in enhancing cold star HC/NOx emission control. We have found that a small amount of Pd (<0.5 wt%) dispersed in zeolite (i.e., BEA) could significantly increase the conversion efficiency of certain HC/NOx species by increasing their release temperature. Pd may also change the release compositions of HC species due to the change in adsorption mechanisms from weak physisorption to strong chemisorption. Laboratory reactor experiments on BEA zeolite with and without Pd are exemplified in this manuscript to illustrate the effect and benefit of Pd-zeolite on capture and release of HC/NOx species such as ethanol, ethylene, propylene, toluene, benzene and isooctane. It was observed in this work that Pd in the zeolite was not stable under high temperature rich conditions due to the sintering of Pd. This indicates a major obstacle of applying Pd-zeolite based HC/NOx traps in a vehicle exhaust gas with a stoichiometric air/fuel ratio. Base metals were added to the Pd-zeolite to stabilize and significantly improve the performance of the Pd-zeolite exposed to high temperature rich conditions. Vehicle emissions test results also confirmed the benefits of stabilized Pd-BEA zeolite in reducing cold start HC emissions.