The vapor-phase structures of the rotamers of ethylenediamine and the composition of the gaseous system at sample temperatures of 343, 463, and 713 K have been analyzed from electron-diffraction data augmented by rotational-constant data taken from the literature and by ab initio calculations. The molecules are predominately in either of two gauche conformations (A and B), each consistent with the formation of an internal hydrogen bond. The mole fractions of the combined gauche forms at 343, 463, and 713 K are 0.882(69), 0.786(96), and 0.812(92). The tendency of the composition to shift toward the anti form as the temperature increases is indicative of a lower internal energy for the gauche form. Analysis of the temperature dependence of the composition gives the energy and entropy differences as Delta E degrees = E(A) degrees - E(G) degrees = 0.68 (sigma = 0.41) kcal/mol and Delta S degrees = S-A degrees - (S-G degrees + R ln 2) = -0.29 (sigma = 0.90) cal.mol(-1).K-1. The following are values for some of the more important distances (r/Angstrom) and angles (angle/deg) for the gauche A, gauche B, and anti forms at 343 K; uncertainties estimated at 2 sigma are given in parentheses : r(N-H) = 0.995(6) (all forms); (r(C-H))1.125(6), 1.124(6), 1.124(6); r(C-C) 1.520(14), 1;526(14), 1.520(14); r(C-N-H-acceptor) = 1.473(5), 1.471(5), 1.469(5); r(C-N-H donor) = 1.466(5), 1.464(5); angle CCNH-acceptor = 110.2(4), 110.3(4), 110.3 (4); angle CCNH-donor - 110.1(4), 115.3(4); (angle CCH) = 108.7(20), 108.8(20), 108.7(20); angle NCCN = 63.3(9), 59.5(10), 180.0 (assumed).