The phase separation of a thin polymer blend layer undergoing a simultaneous transesterification reaction is examined by a variety of experimental techniques [neutron and X-ray reflectivity, optical microscopy, atomic force microscopy (AFM)] to determine morphological features unique to reactive phase separation. Neutron reflectivity measurements suggest; that a copolymer layer reactively forms at the interface between the phase-separating blend components. Evidence for a copolymer layer is also provided by AFM and optical images which indicate a strong inhibition ("frustration") of droplet coalescence and a tendency of the droplets to cluster in late-stage phase separation. The influence of the transesterification reaction on phase separation is found to depend strongly on temperature.