In this study, the durability characteristics of three catalyst systems were evaluated. One system was a conventional platinum-rhodium catalyst. The second was a catalyst formulated with palladium and rhodium as the active components. The third system was a catalyst with palladium as the only active metal. These catalysts were evaluated for emissions performance after aging on two durability schedules. The first was a “3-step” aging schedule which is characterized by three distinct temperature steps from 440°C to 775°C. The second schedule involved steady state aging at 440°C. These aging schedules were set up on water-brake engine dynamometers. After low temperature aging, the palladium containing catalysts did not perform as well as the platinum-rhodium catalysts. After 400 hours aging at 440°C, the palladium catalysts had a steady state CO/NOx conversion efficiency of 77% while the platinum rhodium catalyst had a CO/NOx conversion of 95%. It was also observed that this low temperature aging effect was reversible.