On-chain chemical defects have recently been identified as the source of broad, low-energy emission features in polyfluorenes. By considering the evolution of the time-resolved luminescence spectrum of a dilute solution of a ladder-type poly(p-phenylene) it is shown that on-chain defects also play an important role in the emission from these materials. A broad band centred at 560 nm, which has previously been attributed to aggregate emission, is identified in both solution and film, suggesting an incompatibility with the aggregate picture. This provides an explanation for the recent observation of a decrease of field-induced exciton dissociation with increasing emission wavelength as excitons are spatially more localised on chemical defects than on the polymer backbone. (C) 2002 Elsevier Science B.V. All rights reserved.