The microwave spectrum of the aziridine-borane complex (C(2)H(5)N-BH(3)) has been investigated by microwave spectroscopy in the 18-80 GHz spectral region. The spectra of the ground vibrational state and three vibrationally excited states have been assigned, and the vibrational frequencies of these states have been determined. The complex was found to have a symmetry plane (C(s) symmetry) formed by the N-B bond and the bisector of the aziridine ring. The dative N-B bond is found to be as short as 161.5 pm in MP2/aug-cc-pVTZ calculations. The MP2 structure of the aziridine ring of this complex is nearly the same as that in the substitution structure of aziridine. Complex formation therefore appears to have little influence on this moiety. The dipole moment was determined to be mu(a) = 17.72(11), mu(b) = 0.0 (by symmetry), mu(c) = 5.70(8), and mu(tot) = 18.62(11) x 10(-30) C m [5.581(34) D]. The barrier to internal rotation of the borane group was determined to be 12.1(3) kJ/mol using the microwave splitting method. The microwave investigation has been augmented by high-level quantum chemical calculations at the MP2/aug-cc-pVTZ and B3LYP/6-311++G** levels of theory. There is generally good agreement between the experimental results and quantum chemical predictions.