The densities, d, and fluorescence spectra of ternary mixtures of water (1) + dimethyl sulfoxide (2) + lysozyme (3) were measured, and the partial specific volumes of lysozyme, v̄°, were determined as the extrapolated values of the apparent partial specific volumes at an infinitely dilute concentration of lysozyme at 298.15 K. A binary mixture of water + dimethyl sulfoxide has a minimum excess volume when the mole fraction of dimethyl sulfoxide (x 2) is in the range 0.25 to 0.42, and lysozyme unfolds and aggregates within this range. This indicates that the strong interaction between water and dimethyl sulfoxide might influence the conformation of lysozyme by interfering water−lysozyme interactions. In the range x 2 > 0.8, lysozyme was in a disaggregated−denatured state, having a partial specific volume of (0.706 ± 0.002) cm3·g-1, which was smaller than that in the native state, (0.712 ± 0.001) cm3·g-1, probably because the unfolding led to a decreased cavity volume of lysozyme in the solvent.