Preferential oxidation (PROX) of CO in H2-rich stream is critical to the production of clean H2 for H2-based fuel cells. A low-temperature fuel-cell start up is desirable for transportation applications. However, few catalysts are able to provide the required sustained activity at near-room-temperature conditions. The development of highly active and selective PROX catalysts is highly desirable but has been proven to be extremely challenging. Pt nanoparticles with a size of (1.2±0.1) nm were synthesized in situ on 32 nm magnetite nanoparticles, which were previously deposited on alumina supports. The resulting catalyst was found to be highly active towards the preferential oxidation of CO in a H2-rich reformate stream and gave 100 % CO conversion at 25 °C with a weight hourly space velocity of 0.5 mL min−1 mg−1 and an oxygen excess factor (λ) of 2. [ABSTRACT FROM AUTHOR]