Operando measurements were carried out in a quartz reactor to evaluate the catalytic performance of NOx storage and reduction materials containing Pt and Ba supported on Al2O3. Carbonates present on the surface after activation were removed after the first exposure of the sample to the nitration flow. Nitrite species bound to barium were observed at low temperatures. Barium nitrates are the predominant species in the studied temperature range under wet and dry conditions. These species are not stable at temperatures above 723 K. The presence of water inhibits the formation of alumina nitrates favoring the coordination of nitrates with the barium sites. (c) 2006 Elsevier B.V. All rights reserved.