The behaviour of the velocity and the change in the absorption of longitudinal ultrasound in the process of structural relaxation is investigated at a frequency of 50 MHz using ultrasonic spectroscopy in the temperature range 77–300 K in the Mg–5% Sc alloy deformed by equal-channel angular extrusion. Peaks of acoustic absorption with localization temperatures of $\sim$232 and 190 K are found. An estimate of the activation energy ($\sim$0.5 and $\sim$0.16 eV) indicates that these relaxation processes are due to dislocation relaxation resonance. The influence of the kinetics of the processes of structural relaxation in the extruded alloy on the investigated acoustic characteristics is shown. As found, that the evolution of the temperature spectrum of acoustic absorption in the Mg–5% Sc alloy is caused by the return of the structure after severe plastic deformation.