The molecular parameters of carbon suboxide, C3O2, have been determined in large-scale ab initio calculations using the coupled-cluster method, CCSD(T), and basis sets of double- through quadruple-zeta quality. The potential energy function for the large-amplitude CCC bending motion (the v(7) mode) was determined to be strongly anharmonic. The equilibrium structure of the molecule was found to be bent, with a barrier to linearity of only 18 cm(-1). The rotation-bending energy levels were then calculated using a semirigid-bender Hamiltonian. The vibrational energy levels and effective rotational constants determined for various v(7) states were found to be in good agreement with the experimental data.