From measurements on pure rotational transitions in the microwave region the moments of inertia, $I_B$ in g ${\mathrm{cm}}^2$×${10}^{-40\mathrm{}}$, have been determined as follows: 32.8544 for ${\mathrm{C}}^{12}$${\mathrm{H}}_3$F, 33.7444 for ${\mathrm{C}}^{13}$${\mathrm{H}}_3$F, 81.0693 for ${\mathrm{C}}^{12}$H${\mathrm{F}}_3$, and 107.286 for ${\mathrm{P}}^{31}$${\mathrm{F}}_3$. The molecular dimensions determined are: for C${\mathrm{H}}_3$F, $d_{\mathrm{CH}}=1.109\mathrm{}\mathrm{A}$, $d_{\mathrm{CF}}=1.385\mathrm{}\mathrm{A}$, and ∠HCH = 110° 0′; for CH${\mathrm{F}}_3$, $d_{\mathrm{CF}}=1.326\mathrm{}\mathrm{A}$, with $d_{\mathrm{CH}}=1.111\mathrm{}\mathrm{A}$ (assumed) and ∠FCF=110° (assumed); for P${\mathrm{F}}_3$, $d_{\mathrm{PF}}=1.546\mathrm{}\pm 0.008\mathrm{A}$ with ∠FPF = 104±3° (assumed). The line breadth parameters, $\Delta \nu$, normalized to 1 mm of Hg pressure are for C${\mathrm{H}}_3$F 20 mc, for CH${\mathrm{F}}_3$ 18 mc, and for P${\mathrm{F}}_3$ 16 mc. Confirming evidences for the nuclear spin values, ½ for ${\mathrm{P}}^{31}$ and ${\mathrm{F}}^{19}$ have been obtained.

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