The molecular complexes formed between sodium atoms and 12-crown-4 molecules were generated in a flow reactor and studied with photoionization mass spectroscopy. The photoionization threshold energies of these complexes, which are dependent on the complex conformations, were obtained from the analysis of the photoionization efficiency spectra. Ab initio molecular orbital calculations were performed on the possible conformers of these complexes and their corresponding ionic states. With the help of theoretical calculations, the photoionization threshold energy of the C-4 conformer of Na(12-crown-4) was determined to be 3.01 +/- 0.02 eV, and the C-s conformer to be 3.36 +/- 0.04 eV. The photoionization threshold energy of the Ci conformer was estimated to be 3.82 +/- 0.04 eV. The conformation-dependent valence electron spatial distributions, bond dissociation energies, and photoionization threshold energies of the Na(12-crown-4) complex could be rationalized and classified in the framework of the general properties of the complexes formed between the sodium atoms and Lewis base molecules.