Photoelectronic responses of organic-inorganic hybrid perovskite CH 3 NH 3 PbI 3 on mesoporous TiO 2 electrodes are investigated. On the basis of near-band-edge optical absorption and photoluminescence spectra, the bandgap energy and exciton binding energy as a function of temperature are obtained. The exciton binding energy is much smaller than thermal energy at room temperature, which means that most excitons are thermally dissociated, and optical processes are determined by the photoexcited electrons and holes. We determined the temperature dependence of exciton binding energy, which changes from ~30 meV at 13 K to 6 meV at 300 K. In addition, the bandgap energy and the exciton binding energy show abrupt changes at 150 K due to structural phase transition. Our fundamental optical studies provide essential information for improving the device performance of solar cells based on halide perovskite semiconductors.