The molecular complexes formed between potassium and ethylenediamine (en), K(en) and K2(en), were generated in a flow reactor and studied by photoionization mass spectrometry. The photoionization efficiencies of these complexes were analyzed with Watanabe plots, and the photoionization threshold energies were determined. Ab initio molecular orbital calculations were also performed for the neutral complexes and their corresponding positive ions. Supported by the calculation results, several conformers of K(en) in the photoionization efficiency spectrum were assigned. The photoionization threshold energies were measured to be 3.34, 3.54, and 3.64 ± 0.01 eV for three types of K(en) conformers and 3.57 ± 0.05 eV for the cyclic conformer of K2(en), respectively. The upper bound of the enthalpy of the complex dissociation of K2(en) into K2 and ethylenediamine was determined to be 10.4 ± 1.5 kcal/mol. The conformational analysis of the potassium‒ethylenediamine molecular complexes using photoionization spectrometry was reported for the first time. On the basis of the molecular orbital study, the structures and the nature of the bonding of these molecular complexes were also described.