A detailed spectroscopic study of the photochemically induced electronic-to-electronic energy transfer reported by us earlier (Gudipati, M. S. J. Phys. Chem. A 1997, 101, 2003) in O ... CO Seminate van der Waals complexes in Ar matrices revealed that these complexes are stabilized in two distinct sites, a singly substitutional (SS) site and a doubly substitutional (DS) site. Ln the SS site the spectra of O and CO are strongly perturbed, whereas in the DS site spectroscopic features nearly identical to the isolated O and CO have been observed. Theoretical calculations on the ground-state O ... CO complex in the SS and DS sites of the Ar lattice are in agreement with the experimental observations. Photolysis of CO2 in Xe/CO2/Ar matrices leads to the formation of Xe ... O ... CO complexes, and energy transfer from the excited CO to XeO that results in emission from XeO has been observed. Electronic spectra corresponding to the O ... CO complexes in Kr matrices have also been observed, but only as weak features superposed on strong charge-transfer transitions in KrO in Kr matrices. We proposed to unify the nonresonant excitation energy transfer processes that involve the formation of an excited species under photochemically induced energy transfer.