Although carbazole-containing copolymers are frequently used as hole-transporting host materials for polymer organic light-emitting diodes (OLEDs), they often suffer from the formation of undesired exciplexes when the OLED is operated. The reason why exciplexes sometimes form for electrical excitation, yet not for optical excitation is not well understood. Here, we use luminescence measurements and quantum chemical calculations to investigate the mechanism of such exciplex formation for electrical excitation (electroplex formation) in a carbazolepyridine copolymer. Our results suggest that the exciplex is formed via a positively charged interchain precursor complex. This complex is stabilized by interactions that involve the nitrogen lone pairs on both chain segments. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012