Platinum nanocrystals, with an average diameter of 12 nm, were prepared by reduction of K2PtCl4 with H-2 in the presence of a structure-directing agent (polymer of N-isopropylacrylamide). The Pt nanocrystals, having mainly (100) structure (approximate to64%), were supported on gamma-Al2O3 and then tested for the reduction of NO (1%) with CH4 (0.6%) in the temperature range 300-500 degreesC. The NO was completely converted to N-2 and N2O starting from 400 degreesC. The formation of NH3 as side product could not be evidenced below 600 C. In the temperature range investigated, the only product of methane oxidation was CO2. The highest selectivity to N-2 (approximate to73%) was observed at 500degreesC. In contrast to Pt(1 0 0)Al2O3 catalyst, substantial amounts of CO and NH3 are formed over the conventionally prepared Pt/gamma-Al2O3 catalyst (NH3 starts to be formed at temperatures as low as 400 degreesC). The remarkable differences observed between Pt(1 0 0)/Al2O3 and Pt/Al2O3 (conventional) catalysts reveal the importance of the morphology of the supported Pt particles for NO/CH4 reaction. Both, the size and dominant crystallographic orientation of the supported Pt particles have been found to be determining factors for the catalyst activity and selectivity. The formation of NH3 and CO can be suppressed by using as catalyst the large, well-defined Pt nanocrystals supported on alumina.