We report the discovery of soliton-like electrogenerated chemiluminescence (ECL) waves in the electrochemical oxidation of thin films (15-250 nm) of the semiconducting conjugated polymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). The ECL "wave fronts" were visualized and imaged in space and time by optical emission microscopy and observed to freely propagate parallel to the plane of the electrode. The data reveal that the EC solitons are triggered by the flux of electrolytes into the polymer film at nanoscale "leaks" (or defects) and propagate by a highly nonlinear, positive-feedback kinetic mechanism involving EC oxidation, mass transport of electrolyte anions, and oxidation-induced swelling, all taking place within the conjugated polymer film. Several. model systems with spatially distributed reaction "triggers" were studied to elucidate the mechanistic details of this complex oxidation process in polymer films.