Nanopowders of unordered solid solution Pt0.5Co0.5 and metal-oxide composite Pt-CoOx were prepared via double complex salt [Pt(NH3)4][Co(C2O4)2(H2O)2]·2H2O decomposition in He and air streams, respectively. It was shown that, compared to metallic Pt nanopowder, both Pt0.5Co0.5 and Pt-CoOx exhibited high CO preferential oxidation (CO PROX) performance under mild conditions and provided complete CO conversion with 50–100% selectivity at near-ambient temperature and WHSV of 80,000 cm3 g− 1 h− 1. Operando XRD study revealed that under CO PROX conditions Pt0.5Co0.5 was stable, while Pt-CoOx underwent reduction even at 50 °C and consisted of Pt and Co nanoparticles. The high performance of both (alloyed and not alloyed) Pt-Co bimetallic systems exhibits the key role of Pt-Co interface, i.e. the so-called “ensemble effect”, in the synergism.