Future emission standards for passenger cars are mainly aiming at a stringent reduction of their hydrocarbon (HC) emissions. A key factor to meet these requirements for passenger cars with otto engines and closed-loop three-way catalyst is the improvement of the cold-start behavior of the aftertreatment device. Amongst other concepts HC-adsorber systems have been proposed to cope with this problem.In the present paper, results of a fundamental research program on these molecular sieve adsorber systems are discussed.Model gas reactor experiments were used to select raw materials for hydrocarbon-adsorption capacity. The materials of choice were used either alone or in combination with state-of-the-art three-way catalysts; the performance of these systems was evaluated on two different vehicles according to the FTP 75 cycle. To get quantitative information about the nature of the stored HC, all investigations were supported by a detailed gas chromatographic HC-analysis.It is demonstrated in this study that for well-tuned aftertreatment systems, the introduction of HC-adsorbers allows to achieve a reduction of the HC-emissions during the FTP 75 cycle of 50% as compared to three-way catalysts alone.