Aqueous solution densities were measured at 25 degrees C for octanoyl-, nonanoyl-, and decanoyl-N-methylglucamides and for their binary mixtures by means of an oscillating-tube densimeter; their (mean) partial molar volumes (V) were determined. The V plotted against the molality of single surfactant systems was characterized by a sharp increase at the critical micelle concentration (cmc), followed by a slight increase, and then it reached a concentration-independent value at concentrations higher than ca. twice the cmc : the infinite dilution values (V-0), the micellar partial molar volumes (VM), and the volume changes accompanying micelle-formation (Delta V = V-M - V-0) were evaluated. The Delta V per hydrophobic CH2 group was 0.8 cm(3)/mol, which was in good agreement with that for some other surfactant systems Moreover, the negative values of V were in accord with the excess partial molar volumes of CH2 groups in alkanols. The volumetric behavior of every two-surfactant mixture was similar to that of single surfactant systems, except that the V continued to increase slightly over a wide range above the mixed cmc. The V-0 and VM of surfactant mixtures were also determined : these values agreed very closely with the sum of the products (mole fraction of individual surfactant in a surfactant mixture) x (V-0 or V-M in single surfactant solutions). On the other hand, the concentration dependence of V was theoretically examined. For this purpose, the concentrations and compositions of monomeric Surfactants and mixed micelles were calculated, on the basis of a theory developed by Clint, and, in addition, the partial molar volumes of every surfactant in mixed micellar solutions were assumed to be equal to those in its pure micellar solutions. The V values thus calculated were in good agreement with the observed ones in the whole concentration range studied.