Photodegradation processes of carbocyanine dyes spin-coated on a polycarbonate plate were studied by steady-state photolysis and near-IR emission spectroscopy. The efficiency of the whole photodegradation caused by molecular oxygen directly and/or indirectly was found to strongly depend on both substituents on the 1 and 1' positions and the counterion. Singlet molecular oxygen ((1)Deltag) produced at the interface between the dye thin film and a polycarbonate plate was detected by near-IR emission spectroscopy. Its yield did not depend on substituents on the 1 and 1' positions but on the counterion. The relative reactivity of carbocyanine dyes in the film state with the singlet molecular oxygen was estimated by exploiting a perinaphthenone thin film as a sin-let molecular oxygen generator. The reactivity strongly depended on both substituents on the 1 and 1' positions and the counterion.