The electronic ground state and the first valence excited states of diacetylene (3Σ+u, 1,3Δu, 1,3Σ-u) are investigated with the aid of ab initio methods applying basis sets of double-zeta and triple-zeta + d quality. SCF, SCF CI, and MC SCF CI methods have been applied. We report equilibrium geometries, vertical and adiabatic excitation energies, and discuss two-dimensional bending potentials in detail. Both C2h- and C2v-type structures are considered. For the lowest singlet states harmonic vibrational analysis of totally symmetric modes has been carried out. Previous experimental assignments of singlet excited states of diacetylene are essentially confirmed in this study and new results on the energetics of triplet excited states are reported.