Dynamics of thermally induced composition redistribution of sandwich-like Pt-Ni octahedral electrocatalysts was studied in situ using a variable-temperature environmental transmission electron microscope (ETEM). Dislocations were observed and on heating treatment moved toward the surface over time, resulting in an alloy nanoparticle with significant increase Ni content on the surface, which was confirmed by X-ray photoelectron spectroscopy. The thermally treated Pt-Ni octahedral catalysts showed significant improvement in catalyzing oxygen reduction reaction over the as-made nanoparticles, reaching values of 1.4 A/mg(Pt) in mass activity and 4.8 mA/cm(Pt)(2) in area specific activity. Our study shows the important effect of postsynthesis treatment on the optimization of Pt and Ni atomic distribution, thus the catalytic activity and stability; and the power of in situ ETEM in understanding the structural origin of enhanced catalytic performance with atomic level of details. (c) 2015 American Institute of Chemical Engineers AIChE J, 62: 399-407, 2016