Rovibrational states of the ground electronic state of Si 2 12C and Si 2 13C isotopomers have been calculated variationally. The potential energy surface used in the calculations was obtained from an MP2/TZ2Pf ab initio surface of Barone et al. (V. Barone, P. Jensen and C. Minichino, J. Mol. Spectrosc., 154 (1992) 252) by applying the restrictions of 100° ⩽ α < 150°. The ab initio surface was refitted to a fourth-order polynomial with an Ogilvie-Tipping variable using a multi-dimensional least-squares procedure. The force field was then embedded in an Eckart-Watson vibration-rotation Hamiltonian, from which low-lying vibrational states and rovibrational states of Si 2 12C and Si 2 13C were obtained. The calculated vibrational states (100) and (001) of Si 2 12C and the 13C isotopic shifts agree well with a recent experiment (J.D. Presilla-Marquez and W.R.M. Graham, J. Chem. Phys., 95 (1991) 5612). Also, the calculations support the vibrational transition at 658.2 cm −1 found by Kafafi et al. (Z.H. Kafafi, R.H. Hauge, L. Fredin and J.L. Margrave, J. Chem. Phys., 87 (1983) 797). The rotational energies of these isotopomers for the lowest six vibrational states are given as are the rotational constants for Si 2 12C and Si 2 13C.