The vibrational dynamics of lutetium orthophosphate (LuPO4) single crystals was carefully investigated by means of polarized micro-Raman spectroscopy and ab initio calculations. Eleven of the twelve independent components of the polarizability tensor, expected on the basis of the group theory for LuPO4, were selected in turn and assigned in symmetry. The only B1g(2) Raman mode was not observed, likewise due to either its very small intensity or its nearness in energy with forbidden Raman modes, which spill and could hide it. Both Raman and infrared vibrational modes are evaluated by density-functional theory calculations using effective core pseudo-potential. The agreement between calculated and experimental frequencies is very good. On the basis of our ab initio results, and of reduced-mass ratio considerations, the expected wavenumber of the missing B1g (2) mode falls close to that of Eg(3) mode peaked at about 306 cm−1, and therefore we can definitively conclude that the observation of the missing B1g (2) mode is masked by the spill-over of this Eg mode. Copyright © 2013 John Wiley & Sons, Ltd.