Optical dephasing of the 5388 Å transition between the lowest Kramers doublets of the 4S 3 2 and 4I 15 2 multiplets of Er3+:LaF3 has been studied by photon echo, optical phase switching and optical free induction decay. Er3+-19F hyperfine interactions produce dephasing which is two orders of magnitude faster than in previously studied non-Kramers systems, but at high field changes in the spin dynamics result in microsecond dephasing. For the lower Zeeman component of 4S 3 2, T2 (=6μs) is independent of H0 whereas for the upper component the dephasing is rapid and strongly field dependent. This is quantitatively accounted for by spin lattice relaxation of the upper component of 4S 3 2. Below 20 kG concentration and temperature dependent dephasing due to electron spin diffusion is observed. © 1982.