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An adsorber system for reducing cold start hydrocarbon (HC) emissions has been developed combining existing catalyst technologies with a zeolite-based HC adsorber. The series flow in-line concept offers a passive and simplified alternative to other technologies by incorporating one additional adsorber substrate into existing converters without any additional valving, purging lines, or special substrates. This contribution describes the current development status of hydrocarbon adsorber aftertreatment technologies. We report results obtained with a variety of adsorber, start-up, and underfloor catalyst system combinations. In each case, it was possible to achieve HC emission levels in compliance with the ULEV standards, and in the best cases, demonstrating HC emissions substantially below the legislated standard.