TheE' block of the mean-square amplitude matrix (Σ) for the planar symmetrical XY3molecular model is treated theoretically in terms of the Keating coordinates. A numerical example for11BF3is given. Also the mean amplitudes of vibration (ι) and Bastiansen-Morino shrinkage effect (Δ) are considered. New relations which connect the mean-square amplitudes and force constants are given. Previous works on mean amplitudes and related quantities for boron trihalides from vibrational spectroscopy and gas electron diffraction are reviewed, and some new calculations are reported.