Density functional theory (DFT) calculations have been used to investigate the process of dimerization for the three chalcogen diimides MeN=E=NMe (E = S, Se, Te). DFT calculations for these monomers reveal that the energies of the syn,syn and syn, anti isomers differ by <3.5 kT mol(-1) for all three chalcogens while the anti,anti isomers are substantially higher in energy (by 25-39 kJ mol(-1)). A qualitative understanding of this difference can be derived from consideration of the electronic structures of the chalcogen diimides. In particular, the antibonding interaction between the in-plane nitrogen lone pairs and the pr orbital on sulfur destabilizes the most sterically favorable anti,anti isomer. The calculated dimerization energies for MeN=E=NMe show that the process is endothermic (Delta E = 34.9 kJ mol(-1)) for E = S, approximately thermoneutral (Delta E = -2.8 kT mol(-1)) for E = Se, and strongly exothermic (Delta E = -82.9 kJ mol(-1)) for E = Te. A qualitative analysis of the orbital interactions involved in the dimerization process reveals that the LUMOs of the diimide monomers are populated in a stabilized bonding LUMO-LUMO interaction that is lower in energy than the antibonding HOMO-HOMO interaction. The most significant contribution to the energy of dimerization is the energy required to distort the planar diimide monomer into half elf the butterfly dimer.