The selective detection of methanol by photoluminescence under environmental conditions has been a great challenge for materials science. Herein, a reversible, turn-on-type photoluminescence triggered by methanol vapor in square-planar palladium(II) and platinum(II) complexes, newly prepared from [MCl2(1,3-bis-(diphenylphosphino)propane)] and L-cysteine, is reported. Both the "turn-on" and "turn-off" states of the complexes were crystallographically characterized, which revealed the presence of intermolecular OH center dot center dot center dot O and CH center dot center dot center dot pi interactions between methanol and the complex molecules in the turn-on state. These interactions prevent the vibrational quenching of the luminescence, leading to the turn-on-type luminescence in this system.