The study of changes in the dynamic Young’s modulus of a typical model superplastic Bi–43 wt.% Sn alloy under the conditions of plastic deformation, under which polycrystalline materials are subjected in order to create a structural–phase state capable of manifesting the effect of superplasticity, is performed. Changes in the Young’s modulus are also studied during long-term exposure at room temperature and normal atmospheric pressure, as a result of which the phenomenological indicators of the superplastic flow of the studied alloy are noticeably reduced, but the manifestation of the effect of superplasticity is observed. Acoustic measurements are carried out using the method of a two-component piezoelectric vibrator. An increase in the dynamic Young’s modulus as a result of compression by ≅ 70% on a hydraulic press and in the ageing process is found in both cast and compressed samples. The obtained experimental data are analysed taking into account previously obtained data on changes in the phase composition of the alloy under experimental conditions. The results of the analysis show that the increase in the Young’s modulus as a result of compression is caused by the appearance of internal stresses in the material. The increase in the Young’s modulus during ageing is primarily related to the transition of the alloy from the initial metastable state to the phase state, which is in equilibrium at room temperature. On the kinetic dependences of the modulus of elasticity in both cast and compressed samples, there is an inhibition of its changes at the ageing stag, when the phase equilibrium in the alloy has not yet been established. This is explained by the change in the kinetics of the decomposition of the α(Sn)-phase (a supersaturated solid solution of bismuth in tin) caused by the appearance of phase stresses associated with the volume effect of phase transformation. As shown, such stresses have an inhibitory effect on the progress of decomposition.