The effect of the platinum particle size on the selective catalytic reduction of NOx with C3H6 over Pt-beta has been investigated. The structure sensitivity of the lean deNO(x) reaction towards the platinum phase has been confirmed and a linear correlation between the TOF and the platinum particle size has been derived. Based on our results, it seems that key elements of structure sensitivity in HC-SCR are the NO dissociation on Pt(1 0 0) planes and Pt-O clean-off by the hydrocarbon. These steps are more easily performed on large Pt particles, i.e., in Pt-catalysts with low metal dispersions. On the other hand, the independence of the Pt(1 0 0)/Pt(1 1 1) ratio with particle size explains the similar N-2 selectivity presented along all the samples (similar to35%). (C) 2003 Elsevier Science B.V. All rights reserved.