Abstract The phosphorus content in engine oil additives or biodiesel once combusted can lead to phosphorus deposits on close-coupled catalysts and a subsequent irreversible deactivation. Inductively Coupled Plasma – Atomic Emission Spectroscopy (ICP-AES) analysis of catalyst samples showed phosphorus depositions on a lean NOx trap (LNT) that was aged in a car endurance run. Three new LNTs out of the same serial production process were poisoned by a wet impregnation method with ammonium dihydrogen phosphate solution ((NH4)2HPO4), based on the detected phosphorus concentrations. Solely poisoned as well as poisoned and hydrothermally aged LNTs were tested on a synthetic-gas-test-bench. Light off tests were used to determine the poisoning effect on the oxidation behaviour in lean gas and on rich gas conversion. NOx storage tests were done to investigate the impact on the nitrate formation ability of the LNT. The poisoned samples were compared to a hydrothermally aged LNT at 750°C for 20 hours – an often-used aging treatment in literature – and 110 hours at 750°C as well as to the catalyst from the car endurance run. After wet impregnation with (NH4)2HPO4 no significant aging effect on Light Off, water gas shift light off and NOx storage was noticeable. Additional hydrothermal aging for 20 hours at 750°C resulted in a deactivation in lean gas light off and a strong deactivation in water gas shift light off. A degradation in NOx storage was identified whereas the aging effect through hydrothermal aging was stronger than the impact of phosphorus poisoning. Phosphorus had a deactivating effect on the LNT performance but the concentrations found on the car endurance run aged LNT were identified as not critical. Higher concentrations would have led to a much stronger deactivation and should be avoided. The wet impregnation method with ammonium dihydrogen phosphate was identified as a proven method to selectively deactivate specific LNT mechanisms.