The experiments of Brown, Woolhouse & Valdre (1968) on coherency loss induced by fast neutron irradiation have been extended by varying the precipitate size distribution, the fast neutron dose and the mismatch of the precipitate. When the integrated dose and the mean precipitate size are large, coherency loss is extensive during irradiation. Otherwise thermal activation is required for coherency loss. This is shown to be consistent with a process involving the unfaulting of interstitial dislocation loops and their subsequent glide and climb to the precipitate interfaces. Closer agreement is found with the theoretically predicted critical size for coherency loss of Brown et al. (1968). Experiments involving coherency loss by electron irradiation in a high voltage electron microscope are also described. It is found that in this case, coherency loss occurs by the migration of free interstitial atoms produced by the irradiation and very close agreement with the theoretically predicted size is found.