Hypoeutectic Sn-Bi solder alloys have melting points in the range of 139–232 °C, dependent on the concentration of Bi in the alloy. Typical solidification structures of these alloys are characterised by Sn-rich dendrites surrounded by the lower melting point eutectic Sn-Bi phases. Furthermore, as the solubility of Bi in Sn varies significantly with temperature, depending on the composition, Bi can also be present in the form of precipitates within the Sn-rich phases. The present work uses in-situ synchrotron powder X-ray diffraction to examine how the lattice parameters of Sn and Bi, present in a multicomponent solder alloy based on the hypoeutectic Sn-Bi system, vary in the temperature range of −100 °C to 180 °C. For comparative purposes, the lattice parameters of pure Sn and pure Bi were characterised in the same temperature range. The results confirm the significant influence of Bi dissolution/precipitation that occurs with temperature variations on the lattice parameters of Sn and highlights complexities associated with the multi-length scale microstructure of Sn-Bi alloys.