Three developmental catalytic converters, provided by different companies, were tested at the exhaust of a SI (spark ignition) NG (natural gas) engine for bus application. The catalysts were all based on noble metals: Pt (platinum), Pd (palladium), Rh (rhodium) and differed in size, metal loading and active phase composition. Emission evaluation was performed according to the European ECE-R49 procedure (13 mode cycle), in stoichiometric and lean-burn conditions. In addition to regulated emission measurement, speciation of NMHC (non-methane hydrocarbons) and carbonyl compounds was performed. The results showed that all the catalyst compositions considered allowed the European emission limits to be complied when the engine operated in stoichiometric conditions, while the overall best performance in the lean region was obtained on the catalysts with noble metal composition Pd:Rh=21:1. For NMHC and carbonyl compounds, not limited by regulation, conversion efficiency results always close to 100%. About NOx (nitrogen oxides) emissions in lean operation, the effect of the different noble metals on the catalyst conversion efficiency was investigated and compared with results obtained on cobalt-based de-NOx catalysts.