Carbon-supported PtIr nanoparticles with the nominal atomic ratio of 70 to 30 % in the 3 nm scale are investigated for the ammonia oxidation reaction (AOR) in 1 M KOH. The morphological and surface properties of alloyed PtIr electrocatalysts synthesized at pH 7 and 8.5 are characterized by TEM, XRD, SAXS and XPS. According to SAXS, nanoparticles prepared at lower pH (PtIr (1)) are mono-dispersed with particle size of 2.9 nm, whereas nano-sized catalyst PtIr (2) synthesized at higher pH has bi-modal size distribution with modes at 1.8 and 3.4 nm. XPS revealed that Pt on the surface of PtIr (1) is present in metallic state contrary to PtIr (2) where platinum surface species of higher oxidation state are identified. Iridium on the surface of both samples is present in Ir-0, Ir3+ and Ir4+ form. From the electrochemical characterizations, the onset potential of PtIr for ammonia oxidation is more negative (-1.07 V vs. MSE) compared to the monometallic Pt nanoparticles of 3.0 nm in size (-0.94 V vs. MSE). Long term electrolysis (12 h) demonstrated a 33% higher degradation rate of ammonia on PtIr (1) than on Pt with N-2 as the main product, in addition to some traces of NO3 and NO2. Higher catalytic activity, stability and activity recovery of PtIr nanoparticles is attributed to the electronic effect generated between Pt and Ir atoms in alloy. The surface of alloyed PtIr nanoparticles displays a complex of physicochemical and electrocatalytic properties, thus the maximum electrocatalytic activity towards AOR is highly correlated with the narrow size distribution and lower amount of surface oxygenated species. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.