Internal friction in ultrafine-grained AZ31 magnesium alloy is investigated by resonant ultrasound spectroscopy. It is shown that the internal friction significantly increases at elevated temperatures (greater than or similar to 100 degrees C), and that this increase can be attributed to grain boundary sliding (GBS). The evolution of this phenomenon with grain refinement is studied by comparing the results obtained for an extruded material and for materials after additional one, two, and four passes of equal channel angular pressing. It is observed that the activation energy for diffusive GBS significantly decreases with decreasing grain size, and so does also the threshold temperature above which this internal friction mechanism is dominant. The results prove that the ultrafine-grained AZ31 alloys exhibit diffusive GBS at temperatures close to the ambient temperature, which is an interesting finding with respect to the possible applications of these alloys in superplastic forming technologies.