All-electron ab initio multiconfiguration self-consistent-field (CASSCF) and multireference configuration interaction (MRCI) calculations have been carried out to determine the low-lying states of the molecule GeC. The electronic ground state is predicted to be (3)Pi. Based on the results of the MRCI calculations, the equilibrium distance for the (3)Pi ground state has been computed as 1.842 Angstrom and the vibrational frequency as 827 cm(-1). The ground state is separated from the excited states (3)Sigma(-), (1)Sigma(+), (1)Pi, and (1)Delta by 3552, 5768, 7322, and 8303 cm(-1), respectively. The chemical bond in the (3)Pi electronic ground state has double-bond character. It is polar with charge transfer from Ge to C giving rise to a dipole moment of 1.68 D at 1.84 Angstrom. Thermal functions as derived from the theoretical molecular parameters were used in the calculation of the thermodynamic properties of the GeC molecule from new mass spectrometric equilibrium data. Also, the literature value for the dissociation energy of GeC has been reevaluated. The recommended dissociation energy, D degrees(0)(GeC), and enthalpy of formation, Delta(f)H degrees(298.15)GeC), in kJ mol(-1) are 455.7 +/- 11 and 630.4 +/- 11, respectively.