In situ Raman experiments excited with 244 nm radiation are carried out on Pt/gamma-Al2O3 and PUBa/gamma-Al2O3 catalysts. The addition of Ba allows the normal catalyst to store substantial amounts of NOx as Ba nitrates under lean burn conditions. Dilute amounts of NO and/or SO2 Plus O-2 in N-2 are flowed over the catalysts, with the temperatures and concentrations chosen to simulate the NOx trapping and sulfur poisoning characteristics encountered in normal operation. Adsorbed species observed on Pt/gamma-Al2O3 are nitrite/nitro species and sulfate on gamma-Al2O3 and NO on Pt. On Pt/Ba/gamma-Al2O3, in situ UV Raman spectra showed typical NOx trap behavior: NOx and SOx are stored as Ba(NO3)(2) and BaSo(4) during lean conditions and released at higher temperatures in H-2 flow. Prolonged SO2 exposure eventually deactivates the catalyst for NOx storage. The sulfate purged by heating to 500 degreesC in H-2 is not completely removed from the catalyst, since it can reappear at lower temperature without additional exposure to SO2. On both Pt/gamma-Al2O3 and Pt/Ba/y-Al2O3, NOx species form more quickly than sulfates under our reaction conditions, but the sulfates are more stable and eventually block NOx adsorption, since they occupy the same sites.