Abstract The photoluminescence of K 4UO 2(CO 3) 3 has been studied under conditions of high resolution at cryogenic temperatures. The origin corresponding to the pure electronic transition was located at 4774 Å (20945 cm -1), and it was found that the totally symmetric uranyl stretching mode was coupled to this transition. A progression of four band systems thus resulted, and from an examination of the energies of corresponding peaks in each system, a value of 813 cm -1 for the U-O stretching mode was determined. Two lattice modes (34 and 80 cm -1) and two molecular vibrational modes (205 and 276 cm -1) were also found to couple with the pure electronic transition, thus yielding approximately 15 major peaks in each band system. The 205 cm -1 vibration corresponded to a CO 2- 3 vibration, while the 276 cm -1 vibration was a UO 2+ 2 deformation. The low values obtained for the force constant and totally symmetric stretching frequency of the U-O bond suggested that in UO 2(CO) 4- 3, the uranium atom is bound in a complex species that may be considered as an intermediate between that of a uranyl (UO 2+ 2) and a uranate (UO 10- 8) ion.