The catalytic performance and optical properties of bimetallic nanoparticles critically depend on the atomic distribution of the two metals in the nanoparticles. However, at elevated temperatures, during light induced heating or during catalysis atomic redistribution can occur. Measuring such metal redistribution in situ is challenging and a single experimental technique does not suffice. Furthermore, the availability of a well-defined nanoparticle system has been an obstacle for a systematic investigation of the key factors governing the atomic redistribution. In this study, we follow metal 1 redistribution in precisely tunable, single-crystalline Au-core Ag-shell nanorods in situ, both at a single particle and ensemble averaged level, by combining in situ TEM with in situ EXAFS validated by ex situ measurements. We show that the kinetics of atomic redistribution in Au-Ag nanoparticles depend on the metal composition and particle volume, where a higher Ag-content or a larger particle size lead to significantly slower metal redistribution. We developed a simple theoretical model based on Fick's first law which can correctly predict the composition and size dependent alloying behavior in Au-Ag nanoparticles as observed experimentally.