A Ke pendulum was used to determine the internal friction, Q -1 , of Zn-Al eutectoid alloy (22 wt% Al) in relation to its microstructure. The internal friction for the as-quenched and slowly cooled samples from above the eutectoid temperature, showed an exponential rise with temperature, characteristic to existing phases and microstructure of the samples. The internal-friction value at any particular temperature decreased as the quenched, metastable state α'-phase transformed to (α + β) phases in thermodynamic equilibrium below the eutectoid temperature. The phase transformation was made by giving the prescribed heat-pulses to the as quenched samples. The energy activating the process responsible for the rise of Q -1 with temperature decreased from 0.544 × 10 -19 J for the α'-phase to 0.448 × 10 -19 J for the (α + β) phases at temperatures below the eutectoid temperature. This decrease was attributed to the role of the different phases in the relaxation mechanism responsible for this effect. The internal friction was used to assess the degree of transformation from the high-temperature phase to the low-temperature phase. Results were discussed in terms of Ham's theory of diffusion-limited precipitation of supersaturated solid solutions.