The C-3 . H2O complex has been formed in argon matrices and studied via FTIR spectroscopy. Five vibrational bands due to the complex have been identified and compared with the results from ab initio calculations (HF/6-31G* level) and normal coordinate force constant adjustment calculations. Eight isotopomeric peaks resulting from C-12/C-13 substitution art observed for the asymmetric CC stretching mode at 2052 cm(-1) and lead to the conclusion that the geometry of the complex is planar and asymmetric. Photolysis of the complex at 405.4 nm results in the appearance of two intermediates, transoid and cisoid rotamers of 3-hydroxypropadienylidene (HPD), and two stable molecules, propynal and C3O. The latter two have been observed previously in interstellar space. The existence of the intermediates was established by comparison with ab initio theoretical calculations of the vibrational frequencies and intensities, C-12/C-13 isotopic studies, and photolytic behavior. Photolysis of the cisoid rotamer produces the transoid rotamer, with spontaneous reversion in the dark. The temperature dependence of the kinetic behavior of the dark interconversion process yields a very low activation energy. Ab initio theory predicts a much different value, leading to the suggestion that the solid-state rotamerization process may occur via hydrogen tunneling. The implications of this finding for the production of molecules of astrophysical importance on dust particles in the interstellar medium are discussed.