In a majority of the cases, the structure of a molecule determined in the crystalline state is used to explain the properties of that molecule in the solution phase. To test the general applicability of such an approach, the structure of tert-butylphenylphosphinoselenoic acid has been determined both in the solution phase and in the crystalline state. To determine the structure in the solution phase, vibrational absorption and vibrational circular dichroism (VCD) spectra of the levorotatory enantiomer of tert-butylphenylphosphinoselenoic acid have been measured in CDCl3 solution in the 2000-900 cm(-1) region. The conformations for both tautomeric structures of monomeric (S)-tert-butylphenylphosphinoselenoic acid are investigated using the B3LYP functional with the 6-31G* basis set. Vibrational absorption and VCD spectra are predicted for these conformations at the same level. For the most stable conformation, the geometry, vibrational absorption, and VCD spectra are also predicted using the B3LYP functional with the aug-cc-pVDZ basis set. Similar investigations were also carried out for dimeric (S)-tert-butylphenylphosphinoselenoic acid. A comparison of the predicted spectra with corresponding experimental spectra in CDCl3 solution indicates that (-)-tert-butylphenylphosphinoselenoic acid has the S configuration and exists predominantly as a monomer in one tautomeric structure and one conformation. The ab-initio-predicted specific rotation also confirms that (S)-tert-butylphenylphosphinoselenoic acid exists as a monomer in chloroform solution and that the absolute configuration is S-(-). The structure in the crystalline state has been determined using X-ray diffraction. Although the absolute configuration is fully supported by the X-ray structure, it is found that (S)-tert-butylphenylphosphinoselenoic acid exists in a crystal as an intermolecularly hydrogen-bonded dimer. This observation suggests that one should be careful in interpreting the solution-phase data using crystal structures.