The isomer shifts for both ${\mathrm{Sb}}^{121}$ and ${\mathrm{Sn}}^{119}$ nuclei have been measured in primary solid solutions of the Sb+Sn system. The experimental results show that the variation of isomer shift with concentration is much weaker than that from one insulating compound to another. This indicates that an increase in the number of electrons per atom hardly changes the number of $5s$ electrons.