Correctly-designed heterojunctions between semiconductor polymers allow for efficient operation of polymeric photovoltaic or light-emitting diodes (LEDs). We show via low-temperature, time-resolved photoluminescence measurements that exciplex-type, localised heterojunction states form at interfaces between poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1, 4-phenylenediamine), PFB, and poly(di-octylfluorene), F8, and that these can undergo endothermic energy-transfer to form bulk exciton states. This process has been demonstrated to enable new pathways for charge recombination and exciton dissociation at polymeric heterojunctions. The new system presented here supports the generality of these phenomena. (C) 2004 Elsevier B.V. All rights reserved.