The in-line hydrocarbon (HC) adsorber is a passive after-treatment technology to address cold-start hydrocarbons in automotive engine exhaust gas. A major technical challenge of the in-line HC adsorber is the difference between the HC release temperature of the adsorber and the light-off temperature of the burn-off (BO) Catalyst. We call this phenomenon the “reversed-temperature difference”.To reduce the reversed temperature difference, NGK has proposed a new “In-line HC Adsorber System” which consists of light-off (LO) Catalyst + Barrel Zeolite Adsorber (BZA), with a hole through the center, BO Catalyst and secondary air injection management (SAE 970266).This, our latest paper, describes the evaluation of various adsorbents and the effect of the center hole on the Adsorber BZA.The adsorber system, which had the Adsorber BZA with a 25mm ϕ center hole and adsorbent coated, confirmed 30% lower FTP NMHC emission versus a system with no center hole or adsorbent coating. These two systems had been RAT-A engine aged to an equivalent to 50,000 miles, and tested in the under-floor position on a vehicle equipped with a 3.8 liter engine. Further, adsorption and desorption performance of the adsorber BZAs and heat up rates of the BO Catalysts with different BZA center hole diameters are also discussed.This In-line HC Adsorber System could be one of the potential technologies to meet LEV/ULEV and Stage-3/Stage-4 regulations, without suffering from thermal deterioration of the close-coupled catalyst in the manifold area.