Breakdown processes in polymer dielectrics pose a limitation to energy storage and high-power operation of many electronic devices. These processes have been studied for many decades, and a great deal of progress has been made in developing phenomenological models . Nonetheless, many open questions remain, particularly regarding the development of a single, unified theory to describe all stages of the breakdown process, from initiation, to sub-critical filament advance, to catastrophic breakdown and/or 'soft breakdown' . To develop and validate such a detailed understanding, it is essential to first develop high-speed, spatially-resolved in situ characterization techniques. Here we demonstrate that thermoreflectance imaging [3-5] can provide such a capability for voltage stress-induced defects in self-assembled polymer nanodielectrics (SANDs) films . Temporal evolution of self-heating and material blister formation for soft and catastrophic failure sites in SANDs films are imaged at submicron spatial resolution. This result shows that thermoreflectance characterization techniques provide insight into the breakdown processes determining polymer dielectric performance and reliability.