The volume expansion of sodium with a potassium impurity content of 100, 200, or 500 ppm (mole) was measured by a capillary method. The temperature region covered extended from 60° up to 100°C. (Melting point of pure sodium 97.83°C.) In the temperature region 60° to about 90°C the results, when combined with x‐ray measurements of the lattice constant, agree well with published values and theoretical estimates. The formation energy of a single vacancy was found to be 0.40±0.05 eV, and 0.45±0.05 eV; the formation entropy, in units of Boltzmann's constant, to be 7.2±1.5, and 7.05±1.5 for the 100‐ and 200‐ppm specimens, respectively. Extrapolation of the single‐vacancy concentration up to the melting point yields 10.5±1.5×10−4and 9.3±1.5×10−4, respectively, again for the two impurity contents, 100 and 200 ppm. The data taken between 90° and 97°C show an excess volume expansion over the extrapolated single‐vacancy values. Several possible mechanisms responsible for this behavior are discussed. The starting point of melting (Tc) was found by a careful analysis of the volume‐expansion curves versus temperature and time. A power law holds for the volume expansion in the region 10−6<(Tc−T)/Tc<10−4. The exponential number is 0.66≈2/3, independent of the potassium content of the sodium sample.