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, secondary air, or special substrates. Major technical issues to be resolved for practical use of this system are 1) the ability to adsorb a wide range of HC molecular sizes in the cold exhaust gas and 2) the temperature difference between HC desorption from the adsorber and activation of the catalyst to convert desorbed HCs. This paper describes the current development status of hydrocarbon adsorber aftertreatment technologies. We report results obtained with a variety of adsorber properties, washcoat structures of adsorber catalyst and start-up and underfloor catalyst system combinations. The system was evaluated in FTP tests using a 2.4-liter L4 vehicle. The system reduced up to 60% of cold-start HC emissions beyond the three-way catalyst-only baseline system. This in-line HC adsorber system could be one of the potential technologies to meet ULEV/SULEV and future regulations, without the need for ancillary electrically heated catalyst (EHC) hardware and its associated costs.