Energy transfer in LaF3; Er, Yb is studied by observing the lifetimes of Er3+in excited states and the excitation spectra both at room temperature and at 77 °K. It is found that the energy transfer from Er3+(4S3/2→4I13/2) to Yb3+(2F7/2→2F5/2) is associated with the emission of three phonons of about 350 cm−1, and that the energy transfer from Er3+(2H9/2→4F9/2) to Yb3+accompanied by the absorption of phonons takes place.