We have identified excited-state singlet oxygen as a reactive intermediate in the solid state photo-oxidation of two poly(p-phenylene vinylene) (PPV) derivatives, poly(2,5-bis(5,6- cholestanoxy)-1,4-phenylene vinylene) (BCHA-PPV) and poly(2-methoxy,5-(2'-ethyl-hexoxy)- 1,4-phenylene vinylene) (MEH-PPV). Singlet oxygen is photosensitized via energy transfer from the polymer and undergoes 1,2-cycloaddition across the electron-rich vinyl double bond in the backbone of the polymer resulting in the formation of highly oxidized species such as esters. Volatile carbonyl species are also formed, and, consequently, the film thins. This degradation pathway occurs when the polymer film is exposed in air to light having energy above the bandgap of the polymer, from either a low-pressure UV mercury lamp or an Ar+ laser operating at 514 or 457 nm. Interestingly, the singlet oxygen reaction does not take place in model compounds with similar structures such as trans-stilbene. In an effort to understand which properties of the polymer make it susceptible to singlet oxygen attack, we have studied the effect of conjugation length and side groups on the reactivity of thin polymer films. Specifically, the role of electron withdrawing groups, steric protection of the vinyl double bond, and conjugation length effects have been investigated using both FrIR spectroscopy and semi-empirical computational chemistry calculations.