The existence of gaseous CoF4-a species with an unusually high degree of oxidation of the metal-has been demonstrated by means of Knudsen effusion mass spectrometry and FTIR spectroscopy. The Knudsen effusion mass spectrometry method has been applied to investigate the gas-phase composition over the CoF3(s)-TbF4(s) system and to establish the optimum in situ conditions for CoF4 synthesis. The electron impact mass spectrum of the CoF4 molecule has been obtained. From the infrared spectrum of argon-isolated CoF4 molecules, the Co-F stretching mode has been detected at 767.8 cm(-1).