The hydrogen-bond acceptor properties of progesterone have been investigated in vacuo with theoretical calculations, in solution by FTIR spectrometry and with octanol-water partition coefficients, and in the solid state through the Cambridge Structural Database. When progesterone is in vacuo, the electrostatic potential spreads mon widely than that of O-20 and is more negative by 6.8 kcal mol(-1) around O-3; the O-3 lone-pair electron ionization energy is lower by 0.11 eV, and consequently, the hydrogen-bonded water-O-3 complex is more stable by 2.8 kJ mol(-1) at 0 K. In CCl4 at 298 K, the hydrogen bonding of a phenol to O-3 is more favorable than hydrogen bonding of a phenol to O-20 by 3.1 and 3.6 kJ mol(-1) on the Gibbs energy and enthalpy scales, respectively. In pure hexafluoro-2-propanol, higher basicity and lower steric hindrance allows O-3 to be a triple-hydrogen bond acceptor, while O-20 is at most a double acceptor. The higher hydrogen-bond basicity and coordination number of O-3 compared to those of O-20 explain the octanol-water partition of progesterone. In the solid state, O-3-bonded water molecules exhibit a characteristic network of hydrogen bonds, and we find (i) more frequent hydrogen-bonded contacts to O-3, (ii) longer C-3=O bonds in the complexes, and (iii) shorter and more Linear OH . . .O-3 hydrogen bonds. These in vitro geometric and thermodynamic hydrogen bonding parameters agree well with the hydrogen bonding of progesterone to its human receptor.