Subject of the paper are converters of lean mixtures for SI engines with a good efficiency of NOx reduction. Investigation of new effective catalytic materials having good sorptive proprieties of nitrogen oxides and simultaneously resistant on the thermal destruction and chemical destruction are main aim of the paper. The operating idea of such converters and conditions of their work are also presented. Main objective of the paper is performance of the new reactor of the Pt/Rh-MgO/CeO2/Al2O3 formula. The magnesium oxide MgO is here an absorber of nitrous oxides and has an essential feature, it is resistant on sintering. Such reactor can be the alternative for generally applied converters containing the barium oxide. Experimental results of catalytic converter containing various quantities of magnesium oxide for SI engines with lean mixtures are presented in the paper. Researches involved both laboratory and engine tests. The laboratory researches concern the topography active surface of the reactor with the scanning electron microscopy) method. Size of platinum and rhodium crystallites obtainedfrom the Scherrer dependence analyzing the diffractive spectrum of the X-radiation (XRD) was about 5 nm. Moreover method covering of reactor's active layers on the metal- monolith is presented. Engine researches comprised CO, THC, and NOx conversion for two engines in conditions of lean (A/F=20.0), reach (A/F=14.0), and stoichiometric (A/F=14.7) mixtures. CO, THC, and NOx conversion were stated in the dependence of temperature catalytic process in conditions of the various mixtures.