The phase stability of dimyristoylphosphatidylcholine (DMPC) in concentrations of aqueous dimethylsulfoxide (DMSO) has been investigated through differential scanning calorimetry (DSC) and fluorescence spectroscopy with 1,6-diphenyl-1,3,5-hexatriene (DPH). The phase transitions temperatures of DMPC were found to increase linearly with increasing concentrations of DMSO up to mole fraction X=0.15 DMSO/H2O. After X=0.15, the pre-transition peak started to merge with the main transition. Simultaneously, the subtransition peak began to disappear as its transition temperature also decreased. At X=0.22, the absence of both the subtransition and pre-transition resulted in the direct transition between the planar gel to the liquid-crystalline phase. Transition enthalpies indicated that the peak of stabilization for the sub-, pre-, and main transition was at X=0.12, X=0.18, and X=0.24 respectively. This demonstrates that DMSO exerts distinct effects on each respective phase and corresponding transition. Fluorescence emission spectra correlated well with the effects of DMSO on the transition temperatures of DMPC seen in our calorimetry data. Initial observations for the subtransition and main transition are further confirmed by fluorescence anisotropy plots that illustrate differences in microviscosity of each phase and how DMSO affects the phase transitions. Our results suggest that the most likely mechanism governing the biological actions of DMSO involves the regulation of the solvation effects of water on the phospholipid bilayer.