High-temperature diffusion affects the long-term stability of superalloys and hence their service life. Yet, details of the mechanisms remain elusive. Here, the γ-γʹ microstructure itself is used as a nanoscale diffusion couple to study the diffusion of alloying elements within narrow and wide γ channels during annealing for up to 100h at 750 °C. These samples are investigated by atom probe tomography which revealed the temporal evolution of the γ stabilizers and the γ՛ formers within the γ channels. A prominent channel width dependence on the compositional evolution is observed where the narrow channels approach equilibrium much faster when compared to the wide channels. The experimentally obtained compositions of the γ channel are subsequently compared to the DICTRA predictions performed for various channel widths. The predicted composition of 10 nm and 75 nm wide channels closely match the composition evolution of the narrow and wide channels respectively, for Co, Cr and Ni while Al showed a much slower transition to equilibrium compared to the DICTRA predictions. These findings highlight, a much-overlooked width dependent compositional evolution in the γ channels that can directly affect the service life of such high-performance alloys.