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Lean NOx traps were tested under various laboratory and engine dynamometer conditions to assess the effects of sulfur, temperature, and other exhaust gas parameters on their NOx efficiency and durability. Most recent vendor supplied NOx trap materials have shown significant temperature durability improvements, but sulfur tolerance has remained unchanged. Trap samples were subjected to sulfur poisoning and were desoxed at high temperatures and at various air to fuel ratios. It was found that a simple sulfate model of sulfur interaction with the NOx trapping specie is not sufficient to explain the sulfur interactions observed.