The dynamics of cleavage of long slender single crystals of LiF has been observed with high‐speed cine´‐photography in order to measure the surface energy and the dislocation processes in a propagating semiplastic crack. The fracture surface energy deduced from these observations could be separated into two parts, a reversible thermodynamic surface energy and a contribution due to dissipative plastic deformation, using the theoretical analysis given in Part I of this series. A specific reversible surface energy of 480±50 erg/cm2was obtained and the plastic contribution was found to persist to fracture velocities > 104cm/sec. Dislocation mechanisms of dissipation persisting to very high crack velocities and to temperatures as low as 90°K were deduced from studies of etch‐pit patterns and atomic step structures on the cleaved surfaces. The atomic step structures were observed by imaging in the electron microscope replicas of eptiaxial metal deposits grown from the vapor at the step sites. The plastic flow processes were found to be consistent with the theory of Part I and the observed magnitude of the plastic contribution to the surface energy.