Methylation of (CF3)3PF2 with tetramethyltin or (CF3)3PX2 (X = F, Cl) with tetramethyllead gave the tetraalkylphosphoranes, CH3(CF3)3PX, which, according to the NMR spectra, are best formulated as five-coordinate phosphoranes rather than isomeric phosphonium salts. NMR (19F, 31P, and 13C) spectroscopic studies at low temperatures consistently indicate that the halogen, F or Cl, occupies an axial position in an assumed trigonal-bipyramidal structure and that the methyl group resides in the trigonal plane giving a ground-state structure with one axial and two equatorial CF3 groups. Metathetical substitution of the halogen with groups Y (Y = OCH3, SCH3, N(CH3)2) using (CH3)3SiY reagents or (CH3)2NH gives the derivative phosphoranes CH3(CF3)3PY which, according to NMR (19F, 3IP) spectroscopic studies at appropriate temperatures, have ground-state structures in which Y and CH3 reside in the equatorial plane of the trigonal bipyramid placing one CF3 group in an equatorial position and two CF3 groups in axial positions. In general CF3 environments are averaged by an exchange process with barriers, as indicated by coalescence temperatures, decreasing in the order N(CH3)2 > SCH3 > OCH3 >> F > Cl. The different CF3 environments in CH3(CF3)3PN(CH3)2 are observed at normal room temperatures. Other mass, NMR, and infrared spectroscopic data are reported as well as the hydrolytic behavior of the phosphoranes.