The delayed fluorescence of poly(p -phenylene vinylene) (PPV) and poly(p -phenylene ethynylene) (PPE) derivative solids and frozen solutions at 20thinspthinspK is described. It provides strong evidence for triplet-triplet annihilation to singlets excitons, accounting for up to {approximately}{number_sign}{number_sign}3{percent} of the total emission in PPV films and {approximately}{number_sign}{number_sign}1.5{percent} in PPE powder. It also yields triplet lifetimes of 70 and 110 {mu}s in PPV films and frozen solutions, and {approximately}{number_sign}{number_sign}200 and {approximately}{number_sign}{number_sign}500 {mu} s in PPE powder and frozen solutions, respectively. {copyright} {ital 1999} {ital The American Physical Society}