Carbon (Vulcan XC-72)-supported bimetallic Pd-Ir catalysts with different Pd/Ir proportions (5-50 mol% Ir, 2 wt% Pd) were prepared by "water-in-oil" microemulsion method (w/o) using solutions of low (0.02 M, L series) and high concentration (0.2 M, H series) of the metals precursors (PdCl2 and IrCl3). The bimetallic particles were examined in terms of nanoscale phase properties (extent of Pd-Ir alloying, phase separation), surface composition (Pd and Ir fractions) and electrocatalytic performance for the formic acid oxidation reaction. Structural characterization was performed using XRD, SEM and HRTEM techniques. Electrochemical characterization allowed estimating the PdH formation ability and the surface composition of Pd-Ir particles what was confirmed by XPS data. The Pd-Ir nanoparticles of similar average size (ca. 4 nm), close to that of Ir (3.8 nm) and below that of Pd (6.2 nm) were formed regardless of the Pd/Ir proportion and the concentration of the metals precursors in the w/o. In contrast to the largely alloyed PdIr nanoparticles with the Pd-rich surface formed at low concentration of the metals precursors (0.02 M), the particles of almost closed surface and bulk Pd/Ir ratios composed mostly of randomly distributed single-phase domains were formed at high concentration (0.2 M). At the lowest bulk Ir content, 5 mol%, the particles have Ir-rich surface regardless of the preparation method. The catalytic studies involving formic acid electrooxidation reaction showed the activity enhancement for the L series catalysts with respect to monometallic Pd/C (twofold TOF increase) and H series counterparts. The Pd85Ir15/C catalyst of the Pd-Ir alloyed and the surface composition expressed by the Pd/Ir atomic ratio near to 6 displayed the highest activity which was 2.9-times higher relative to that of Pd. Graphic abstract