Fluorescence studies of poly(2-methoxy-5-(4-(phenylazo)phenyl-4'-(1,10-dioxydecyl))-1,4-phenylen evinylene) (MPA-10-PPV), a PPV derivative with alkyl-tethered azobenzene side chains that was first synthesized by Yoshino and co-workers, are presented. The emission intensity of MPA-10-PPV in dilute solution is partially quenched by energy transfer from the PPV backbone to the azobenzene side chains, and the energy transfer is more efficient to the cis azobenzene isomer than to the trans. Because of this differential, the emission intensity can be reversibly modulated by controlling the isomeric form of the azobenzene side chain with light. The energy transfer process sensitizes the cis to trans azobenzene isomerization, and this additional isomerization pathway alters the composition of the azobenzene photostationary state in MPA-10-PPV.