Some of the problems encountered in the use of ruthenium as a NOx reduction catalyst are described. The major problem of volatilization in high temperature oxidizing atmospheres has been overcome by providing a stabilizing matrix for the ruthenium. The stabilized catalyst, however, does not have good CO and HC oxidizing performance, such as is required when the NOx catalyst is used for oxidation under cold start conditions. Coimpregnation of Pt and Ru is ineffective in improving this situation, and it was necessary to develop a novel procedure to improve catalyst oxidation activity.Engine dynamometer tests showed that the final catalyst performs well under all of the conditions to which it is subjected. The question which remains unanswered is the durability of the system.