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The relationship between Pt particle size and NO x storage performance has been investigated over a simple model Pt/BaO/Al 2 O 3 and an enhanced lean NO x trap catalyst (LNT). These catalysts were treated at elevated temperature to mimic the effect of thermal aging encountered during the desulfation step in NO x trap catalyst regeneration. A combination of in situ time-resolved X-ray diffraction (TR-XRD), diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) measurements after CO adsorption, and high-resolution transmission electron microscopy (HRTEM) observations of oxidatively thermally aged samples clearly shows the sintering behavior of Pt crystallites as a function of aging time and temperature. At elevated temperatures, Pt crystallite growth occurs rapidly for an initially short period (i.e., 1.5 h) followed by a further gradual increase with time. A direct correlation of NO x storage performance data, obtained after successive thermal treatments of the LNT catalysts under oxidizing conditions, with the results of in situ XRD experiments allows us to clearly attribute performance degradation to observed phase changes. Comparing a simple model Pt/BaO/Al 2 O 3 sample with an enhanced formulation, which displays significantly less Pt sintering, we conclude that Pt crystallite size plays a critical role in determining NO x storage activity. In other words, the retention of small Pt particles after thermal aging is crucial to maintaining high activities. Thus, the prevention of Pt sintering, especially during high-temperature operation, must be a key factor in the design of more durable LNT catalysts.