In situ electron spin resonance (ESR) studies have been performed on composite films consisting of a ruthenium tris(bipyridyl) complex and an inherently conducting polymer, poly(2-methoxyaniline-5-sulfonic acid) (PMAS). The composites were investigated under white light irradiation and potential control conditions to probe photoinduced electron transfer between the ruthenium metal center and the conducting polymer. PMAS exhibited a clear ESR signal, characteristic of the presence of mobile single spin polarons within the polymer structure. Irradiation of the PMAS in the presence of the ruthenium metal center resulted in the photo-oxidation of the Ru2+ to the Ru3+ state, as a result of which the PMAS ESR signal was replaced by a response typical of the Ru3+ salt. Upon removal of the illumination, reversible photo switching occurred. This reversibility makes these novel composites promising for applications in areas such as chemical sensors, light switching, and light harvesting devices.