We explore the photodegradation mechanisms in functionalized tetracene (TIPS-Tc) films and how they are influenced by strong exciton−photon coupling in planar microcavities. We demonstrate that degradation of TIPS-Tc films exposed to air proceeds mainly through an oxygen-mediated pathway, assigned to endoperoxide (EPO) formation, whereas degradation in microcavities proceeds through oxygen-independent photodimerization. The aerobic and anaerobic decay mechanisms were found to differ in rate by more than two orders of magnitude. Both the EPO formation and photodimerization proceeded more efficiently in molecules in configurations favorable for the correlated triplet pair (TT) state formation (precursor to the singlet fission) and their immediate surroundings. For the photodimerization, an alkyne dimer is reported as one of the photoproducts, and its optical properties are presented. Strong coupling of TIPS-Tc to resonant microcavities enhanced the photodimerization quantum yield by a factor of 4.2, with the enhancement robust with respect to cavity detuning.