The [((C 6 H 5 ) 3 P) 2 N] + , [(C 6 H 5 ) 4 P] + and [N(CH 3 ) 4 ] + salts of SeF 5 − , SeF 6 2− and SeOF 3 − and CsSeO 2 F were prepared and characterized. Crystal structures were obtained for [((C 6 H 5 ) 3 P) 2 N][SeF 5 ] and [((C 6 H 5 ) 3 P) 2 N][SeOF 3 ] CH 2 Cl 2 . In contrast to oxygen-bridged dimeric TeOF 3 − , the SeOF 3 − anion in [((C 6 H 5 ) 3 P) 2 N][SeOF 3 ] CH 2 Cl 2 is monomeric and represents the first experimentally well determined molecular structure of a monomeric trifluoro-chalcogenite anion. Similarly, [((C 6 H 5 ) 3 P) 2 N][SeF 5 ] represents the first example of a structure containing a well-isolated undistorted SeF 5 − anion. The NMR and the vibrational spectra and their assignments were re-examined and corrected by comparison with high-level theoretical calculations. Whereas the previously published normal coordinate analysis of SeF 5 − is correct, that for SeOF 3 − needs major revision.