The time taken for the lowest T14 excitons in MnF2 to scatter across the Brillouin zone, i.e., to lose all phase memory, has been measured using a technique of resonant optical pumping and monochromatic fluorescence detection. At 1.9 K these very narrow-band excitons retain some phase memory for up to 1 μsec. It is also found that excitons with rather well-defined momenta are created in the decay of exciton-magnon states, but not in the decay of exciton-phonon states. This determines probable decay mechanisms for these two-particle states. © 1973 The American Physical Society.