Singlet exciton fission, a process whereby one high-energy singlet exciton is converted into two lower energy triplet excitons, is an excited state phenomenon with a potential for a huge impact on the efficiency of organic solar cells. As a result, there has been a recent explosion of experimental studies on a variety of molecular materials and fabricated devices. Currently, a detailed microscopic understanding of this process is lacking, partly because there is a lack of direct experimental probes of singlet fission dynamics. This proposal outlines a photo-musr experiment to differentiate between the competing potential mechanisms. Specifically, we will focus on the role, or lack thereof, of charge-transfer (CT). The spatial and temporal resolution of photo-musr enables the quantum mechanical states of the excitons to be probe directly.