We have observed emission from singlet and triplet exciplexes in electron donor (D)-acceptor (A) organic electroluminescent systems. The exciplex singlet-to-triplet fraction appears to be a function of electronic properties of the (D-A) complex molecular components and applied electric field. The fluorescence from exciplex singlets could be eliminated using a heavy atom (Gd)-substituted acceptor molecule allowing the efficient intersystem crossing of singlet into triplet exciplex states. By this a pure exciplex electrophosphorescent diode has been realized. These findings are of importance for the understanding of the exciplex formation mechanism and, thus, for material choice in device applications. (c) 2006 Elsevier B.V. All rights reserved.