<p>Surface segregation of components is a key factor in determining the physicochemical properties and catalytic activity of bimetallic nanoparticles. In this study, computer simulations are used to analyze the metal distribution of Ir-Pd nanoparticles synthesized via microemulsions. Based on the high difference between the reduction potentials, an Ir-core/Pd-shell structure is expected. However, experimental results have shown a higher Ir fraction at the surface (15–23%). The hypothesis is that this unexpected results may be due to differences in nucleation rates. To investigate this, we performed a systematic study on the influence of critical nucleus size on the final nanostructure when the two metals have very different reduction rates. Our aim was to determine the conditions under which Ir can reach the nanoparticle surface. The results confirm that the large difference in reduction rates mainly governs metal segregation, leading to core-shell structures. However, when the concentration is close to the critical nucleus value, a slower nucleation rate results in higher Ir enrichment at the surface. It can be attributed to both a slow homoatomic nucleation rate and to a slow heteroatomic nucleation rate of Ir-Pd. At higher concentrations, this effect disappears as the higher reactant availability facilitates nucleation, resulting in similar metal segregation regardless of the critical nucleus size. Good agreement between experimental and simulation results supports the conclusions of this study.</p>