jats:p The high temperature acoustic loss and elastic moduli of the Ni-based superalloys RR1000, Waspaloy, and Allvacjats:sup®</jats:sup> 718Plus™ were investigated using resonant ultrasound spectroscopy. An exponential increase in acoustic loss was seen in all alloys, which was viscoelastic in character with an activation energy of ∼ 200–400 kJ.moljats:sup−1</jats:sup>, similar to that of the self-diffusion of Ni. The magnitude of the loss was found to be proportional to djats:sup−0.85</jats:sup>, where d is the grain size. Deviations from the high temperature background were seen in Waspaloy and 718Plus around their γ′ solvus temperatures, attributed to increased diffusion during γ′ dissolution. It is suggested that acoustic loss in Ni-based alloys may be used as a means of determining the γ′ solvus temperatures. </jats:p>