Diesel exhaust contains a lower level of hydrocarbons, which serve as the reductant for the NOx catalyst, than gasoline engine exhaust. An investigation was made of several methods for maximizing the performance of NOx catalysts for direct-injection diesel engines. First, the catalysts were given an HC adsorption capability and then their characteristics were tailored to the HC species contained in diesel exhaust. This HC adsorption capability is designed to achieve better utilization of the HC species in diesel exhaust as a reductant. Second, catalyst performance was examined under passive and active conditions. Excellent catalyst performance was obtained under a passive condition, because at high engine loads, NOx catalysts with an HC adsorption capability can utilize HCs adsorbed under low engine load conditions to reduce NOx. By adopting a common-rail system to facilitate post-injection under an active condition, a higher catalyst inlet HC/NOx ratio can be obtained under all operating conditions, resulting in a dramatic improvement in NOx catalyst performance. However, post-injection causes some problems, such as a fuel economy penalty and deterioration of engine durability.