<p>This work investigates the changes in the elastic properties, microstructure, and phase state of the eutectic alloy Bi–43wt%Sn under conditions of superplastic deformation (SPD) at room temperature. Metallographic and X-ray structural studies were carried out on the samples, along with electron probe microanalysis and examination of the deformation relief. The change of the dynamic Young’s modulus E was analyzed in relation to the observed changes in the alloy’s structure and phase state. A non-monotonic change in Young’s modulus was detected under optimal conditions of superplastic (SP) flow. Intensive development of a viscous dislocation–diffusion grain flow was observed, which contrasts with the commonly accepted view that intergranular deformation predominates under superplasticity conditions. The results obtained for the Bi–43wt%Sn alloy are compared with those previously reported by the authors for the eutectic alloy Sn–38wt%Pb. The experimental data indicate that the SP flow of the studied alloys proceeds alongside the decomposition of supersaturated solid solutions and the relaxation of internal elastic stresses of various physical origins.</p>