We present the results of systematic investigation of catalytic activity of unsupported Pd-Pt nanoparticles towards formic acid oxidation. Investigation of unsupported nanoparticles allowed us to address the changes in catalytic activity caused by interactions between Pt and Pd and to exclude the possible influence of the support. While full range of Pt concentration in Pd was covered, we focused on the role of small amounts of Pt on the catalytic activity of Pd nanoparticles. We determined that formic acid oxidation half-wave potential was lower on Pd-Pt nanoparticles containing small amounts of Pt as compared to pure Pd nanoparticles. In comparison to pure Pd, Pd-Pt nanoparticles also exhibited higher formic acid electrooxidation current density observed in cathodic scans during voltammetric experiments. We attribute the changes in the half-wave potential mostly to the changes of surface electronic properties due to interactions between Pt and Pd, and the changes in catalytic current density to interplay between modified electronic properties and changes in surface morphology. (C) 2016 Elsevier B.V. All rights reserved.