This dissertation investigates the degradation of conjugated polymer poly(3-hexylthiophene) (P3HT). Specifically, exposure of polymer solutions to air and light, and aging of thin films in air and dark has been probed. Solutions of either P3HT or blend of P3HT and phenyl-C61-butyric acid methyl ester (PCBM) were mixed in air and light, and also in dark and inert atmosphere. Thin films of these solutions where then spin coated for UV-vis and Raman characterization. Hole only diodes and photovoltaic devices were also realized and characterized for mobility measurements and photovoltaic performance. Characterizations were repeated after aging the thin films in dark and air for two weeks, and then again after a short annealing at elevated temperature. PCBM was found to reduce the magnitude of P3HT degradation. Solutions mixed in dark and argon were found to respond with a reduction of absorption characteristics when annealed. Solutions mixed in light and air were found to respond with an increase in absorption characteristics when annealed.