Infrared and Raman spectra of phenylboron dichloride have been recorded from 1650 to 20 cm.–1. The thirty-one vibrational modes active in these regions complete the assignment of the spectrum in good agreement with C2v symmetry (for Ph35BCl2) and C8 symmetry (for Ph35BCl37Cl). The barrier to rotation about the boron–carbon link is calculated as ca. 45 kcal. mole–1, signifying considerable π-character in the bond. Thermodynamic properties are tabulated over the range 0–500°.