We have synthesized Pt-M (M = Mn, Fe, and Co) nanoparticles (NPs) on carbon nanotube (CNT) supports (Pt-M/CNT) by one-pot sonochemical reactions and investigated their electrocatalytic properties for methanol oxidation reaction (MOR) with emphases on the effects of particle proximity and composition. The NPs are 2-3 nm in size and are uniformly distributed on the CNT surfaces. A series of Pt-Mn/CNT samples with different Pt loadings (33, 57 and 78 wt %) allows the study on the effects of particle proximity. Since the Mn-contents of these samples are very low (<= 8.0 at %) in the NPs, their properties can be approximated as those of 'pure' Pt NPs. Their mass MOR currents do not show any significant variation, but the specific MOR current increases as the distance between NPs is reduced. On the other hand, incorporation of larger amounts of transition metals in the NPs appears to be a more effective way to enhance the MOR characteristics. Pt-Fe/CNT with Fe-content 39.0 at % and Pt Co/CNT with Co-content 46.6 at % both with high Pt-contents are prepared. They show 2-3 fold enhancement in both mass and specific MOR currents compared with commercial Pt/C (60 wt %). (C) 2015 Elsevier B.V. All rights reserved.