In many examples in molecular thermodynamics, spectroscopy can provide physically meaningful information about intermolecular interactions and solution structure. Examples are provided to illustrate the use of IR, UV-visible absorption and fluorescence spectroscopy : (1) to determine thermodynamic properties of hydrogen bonding and other specific chemical interactions, (2) to determine local compositions in solution, (3) to characterize aggregation of surfactants, and (4) to study aqueous solutions at extreme temperatures and pressures up to 400 degrees C.