We observe that a 2-D single crystalline layer of spherical diblock copolymer domains templated by lateral substrate topology melts via a continuous defect generation process, similar to that predicted by Kosterlitz, Thouless, Halperin, Nelson, and Young. The layer of spheres is allowed to order (or disorder) by annealing for 72 h at a given temperature, T, that corresponds to a given N-X(min), where x is the temperature-dependent Flory-Huggins parameter and N-min is the number of mers in the minority block. The structure of the layer is revealed by imaging the ion beam etched film using scanning force microscopy. If N-X(min) > 9, the film is polycrystalline and the system appears to be kinetically trapped. For 9 > N-X(min) > 7.4, we observe that the film is a single crystal with quasi-long-range order and few defects, as expected for the 2-D crystal. As T is increased further (N-X(min) is decreased), we observe that defects are generated, predominantly dislocation pairs and other dislocation clusters with no long-range strain field. Increasing numbers of free dislocations are observed as N-X(min) is decreased further. This dislocation unbinding produces a hexatic phase which has quasi-long-range orientational order but only short-range translational order. At still higher T (N-X(min) = 7.2), the dislocations unbind into +60degrees and -60degrees free disclinations to form an isotropic 2-D liquid of block copolymer micelles. This hexatic to liquid transition occurs at a N-X(min) below that corresponding to the bulk lattice disordering-ordering transition as measured by small-angle X-ray scattering.