Knudsen effusion mass spectrometry has been applied to establish the optimum in situ conditions for the MnF4 synthesis. MnF4 has been generated in the gas phase over the MnF3(s)-TbF4(s) system at 650-780 K in the Knudsen cell. Under the same conditions the supplying gas for the matrix isolation IR spectra was formed. The infrared spectrum of argon-isolated MnF4 molecules has been obtained for the first time. A T-d symmetry is suggested for the MnF4 molecule.