Phase separation of γ' precipitates determines the microstructure and mechanical properties of nickel-based superalloys. In the course of ageing, disordered γ spheres form inside ordered (L12) γ' precipitates, undergo a morphological change to plates and finally split the γ' precipitates. The presence of γ particles inside γ' affects coarsening kinetics and increases alloy hardness. Here we use atom probe tomography to visualize phase separation in a Ni86.1Al8.5Ti5.4 alloy in three dimensions and to quantify the composition of all the phases with near-atomic resolution. We find that γ' precipitates are supersaturated in nickel, thereby driving the formation of γ particles and observe a compositional evolution of the γ particles, which accompanies their morphological change. Our results suggest that by controlling nickel supersaturation we can tailor the phase separation and thereby the properties of nickel-based superalloys.