Platinum and iridium are known to belong to a family of partly miscible metals. On the other hand, these metals are high demanded as active components of the catalysts for a variety of industrially important processes. In the present work, a series of bimetallic Pt-Ir catalysts supported on alumina were prepared using a “single source precursor” concept. The ratio of metals in the alloy was varied from 1:3 to 3:1. The thermal stability and catalytic activity in CO oxidation of the samples were studied in a prompt thermal aging regime. Monometallic Pt/γ-Al2O3 and Ir/γ-Al2O3 samples were used as references. All the studied catalysts were characterized by UV-vis spectroscopy in initial state and after the aging treatment. It was found that the bimetallic nanoparticles being subjected to the high temperature aging at 600 and 800 ℃undergo redistribution with further stabilization in a noticeably more active state. The observed increase in the catalytic activity is explained by an enrichment of the particles’ surface with platinum.