The photodegradation mechanism study has been conducted on poly(2,5-dioctyl-1,4-phenylene-1,2-dicyanovinylene) (C8-diCN-PPV) and poly[2,5-bis(decyloxy)-1,4-phenylene-1,2-dicyanovinylene] (ROdiCN-PPV) to understand the reason behind the faster photodegradation of C8-diCN-PPV which has a lower LUMO. In both polymers, radical superoxide anion mechanism, which is responsible for electron-rich RO-PPVs, is found to be energetically unfavorable for both diCN-PPVs due to diCN substitution. The IR analysis results confirm this and suggest that singlet oxygen (O-2) is the main culprit for photodegradation of both polymers, which cleaves the CC bonds into carboxylic acids. The rates of MW reduction (by GPC) and increase in carbonyl IR absorption intensity are in excellent agreement for both polymers. Phosphorescence study indicates that the faster photodegradation of C8-diCN-PPV is due to intersystem crossing, which helps generate singlet O-2 upon photoexcitation. No phosphorescence was detected in RO-diCN-PPV, suggesting that inefficient intersystem crossing makes RO-diCN-PPV photochemically more stable. This work shows that a small difference in side chain structure can lead to a significant difference in photochemical stability. (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2820-2828