The recently developed refractory metal high entropy superalloys (RSA) have been proposed as novel materials for high temperature service. A key requirement of any material used in such applications is to retain its properties throughout the life of a component. Consequently, it is critical that the microstructures that give rise to these properties are stable across the temperature ranges experienced in service, yet, at present, very little is known about the microstructural stability of RSA. To address this issue, here we report on the microstructural evolution of two alloys from the AlMoNbTaTiZr RSA system following long duration thermal exposures of 1000 h at 1200, 1000 and 800 ˚C. At these temperatures, the initial microstructures were found to be unstable, forming new intragranular precipitates. In AlNbTa0.5TiZr0.5, both Al-Zr hexagonal and Al-Nb rich orthorhombic phases were observed following exposure at 1200 and 1000˚C, whilst additional fine scale phases were found to have formed following exposure at 800˚C. In AlMo0.5NbTa0.5TiZr0.5, the Al-Zr phase was observed to precipitate within the grains following exposure at both 1200 and 1000˚C. A refractory metal-rich secondary bcc phase was observed following exposure at 1000˚C and 800˚C and a Zr-rich phase precipitated at 800˚C. The formation of these additional phases would undoubtedly affect the properties of these alloys, altering their in-service performance. Consequently, these results highlight the need to consider microstructural stability in the future development of RSA and to gain an enhanced understanding of the phase equilibria of these systems.