High entropy alloys (HEAs) typically consist of five or more elements in nearly equiatomic proportions, where it is hypothesized that the large configurational entropy favors the formation of simple crystalline solid solutions. It is observed, however, that HEAs can undergo phase transformations such precipitation, ordering, and spinodal decomposition, as shown here. These nanoscale microstructural heterogeneities have a profound influence on the alloy’s properties. Using atom-probe tomography we investigate nanoscale phase separation occurring in a Al_0.5CoCrFeNi HEA.