The excess chemical potentials, the excess partial molar entropies, and the partial molar volumes in tert-butyl alcohol (TBA)-dimethyl sulfoxide (DMSO)-H2O mixtures were determined. These data, together with previously published excess partial molar enthalpies (Fluid Phase Equilib. 1997, 136, 207) were used to evaluate intermolecular interactions. The TBA-TBA and TBA-DMSO, and DMSO-DMSO interactions were found to be crucially dependent on the composition. The net interaction in terms of chemical potential is very intricate. For example, net interactions of DMSO with a hydrophobic moiety (represented here by TEA) change from attractive to repulsive as the composition changes. This suggests that general discussions of the affinity of DMSO for nonpolar groups (or surfaces) are meaningful only by specifying the composition region. The interactions in terms of enthalpy and entropy are an order of magnitude larger and strongly compensating. Anomalous changes in the enthalpic/entropic interactions and hence qualitative changes in the mixing scheme of the solution, previously described in respective binary TBA-H2O and DMSO-H2O systems, are also apparent in this ternary system. II was found that as the mole fraction, x(D), of DMSO (third component) increases, the transition in mixing scheme occurred at a progressively lower value of x(B). The behavior of partial molar volume indicated that as x(B) increases, the initial increase in the partial molar volume of H2O on increasing x(D), reminiscent to "iceberg formation", diminished. This suggests that existing TEA molecules already made their contribution to the "iceberg formation". The DMSO-DMSO interaction in terms of volume also showed that the transition occurred at a smaller value of x(D) than that for x(B) = 0. The boundary between the two mixing schemes in the present ternary mixture was a straight line in the x(D)-x(B) field, suggesting that the effect of TEA and DMSO on H2O, causing the transition in the mixing scheme, is additive.