Two missing compounds within the binary K-Se system have been synthesized and structurally characterized by means of single-crystal diffraction. β-K2Se2 comprises cell parameters that are in line with predictions made from the series α/β-A2Chx. K2Se4, in contrast, incorporates “Se52–” chains with 50% occupation of the terminal selenium atoms, and has been synthesized on a rather unconventional solvothermal pathway employing successively mercury and ethylene dithiol as solvents.