Disordered alloy and bi-phase PtSn nanoparticles of nominal Pt:Sn ratio of 70:30 atomic % with controlled size and narrow size distribution were synthesized using a single-step polyol method. By adjusting the solution pH it was possible to obtain Pt7Sn3 nanoparticles of various sizes from 2.8 to 6.5 nm. We found that the presence of NaOH in the synthesis solution not only influenced the nanoparticle size, but as it was revealed by XRD, it apparently also dictated the degree of Pt and Sn alloying. Three catalysts prepared at lower NaOH concentrations (C-NaOH < 0.15 M) showed disordered alloy structure of the nominal composition, while the other three catalysts synthesized at higher NaOH concentrations (C-NaOH > 0.15 M) consisted of bi-phase nanoparticles comprising a crystalline phase close to that of pure Pt together with an amorphous Sn phase. These observations are plausibly due to the phase separation and formation of monometallic Pt and amorphous SnOx phases. A proposed reaction mechanism of Pt7Sn3 nanoparticle formation is presented to explain these observations along with the catalytic activities measured for the six synthesized carbon-supported Pt7Sn3 catalysts. The highest catalytic activity towards ethanol electro-oxidation was found for the carbon-supported bi-phase catalyst that formed the largest Pt (6.5 nm) nanoparticles and SnOx phase. The second best catalyst was a disordered alloy Pt7Sn3 catalyst with the second largest nanoparticle size (5 nm), while catalysts of smaller size (4.5-4.6 nm) but different structure (disordered alloy vs. bi-phase) showed similar catalytic performance inferior to that of the 5 nm disordered alloy Pt7Sn3 catalyst. This work demonstrated the importance of producing bi-metallic PtSn catalysts with large Pt surfaces in order to efficiently electro-oxidize ethanol.