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Catalyzed hydrocarbon traps (HC traps) are being developed to provide HC control during the cold start. HC traps are made up of an adsorbent material, such as a zeolite, and a three way catalyst (precious metal based) present on the same monolith to provide trapping and oxidation functions. Traps are typically formulated with a multi-layer washcoat structure with the catalyst layer on top, and the trapping material placed under. The HC traps operate by storing (trapping) the unburned hydrocarbon molecules in the adsorbent material at a lower temperature when adsorption is favored and oxidation does not occur, and subsequently releasing these trapped HCs when the catalyst has heated sufficiently to convert the HCs to carbon dioxide and water. This paper presents the results of a modeling, laboratory and vehicle study on the performance of HC traps. First we present procedure to evaluate trap performance in the laboratory that provides the kinetic parameters for HC adsorption. A mathematical model is presented that can be used to simulate the HC trap. The feedgas to the zeolite trap on vehicle contains both HCs and water, present in the vehicle exhaust. The various HC species and water compete for adsorption on the zeolites. Lab experiments conducted on zeolites to study the competitive adsorption of various HCs and water is presented. Experiments have been conducted where different HCs and water are sequentially adsorbed on the zeolite, and TPD conducted. For modeling, Langmuir isotherm is assumed to represent the adsorption phenomena on the zeolite sites. Exothermic effects of water adsorption on acidic zeolites have been presented. Details on the experiments done to estimate the heat of adsorption of water on zeolites, and the modeling results of water adsorption on zeolites have also been provided. [ABSTRACT FROM AUTHOR]