In a previous paper (W. P. Kraemer, P. Jensen, and P. R. Bunker, Can. J. Phys. 72, 871-878, 1994) we reported the results of an ab initio calculation of the vibronic (i.e., (N=0) ) energy levels of the (\mathrm{CH}{2}^{+})molecular ion in both the (\dot{X}^{2} A{1}) and (\tilde{A}^{2} B_{1}) electronic states. These two electronic states become degenerate ( ({ }^{2} I I) ) when the molecule is linear, and in the vibronic calculation we used the theory that we have given in another paper (P. Jensen, M. Brumm, W. P. Kraemer, and P. R. Bunker, J. Mol. Spectrosc. 171, 31-57, 1995) in order to allow for the effects of the electronic angular momentum in this situation (i.e., the Renner effect). In the present paper we extend the (\mathrm{CH}{2}^{+})calculation to include (N>0) rovibronic energies and we take into account the effect of spinorbit coupling by using the (a b) initio value of the spin-orbit matrix element from W. Reuter and S. D. Peyerimhoff (Chem. Phys. 160, 11-24, 1992). The only experimental data available for (\mathrm{CH}{2}^{+})come from a measurement of the (\nu_{3}) fundamental band of the ground state (M. RΓΆsslein, C. M. Gabrys, M.-F. Jagod, and T. Oka, J. Mol. Spectrosc. 153, 738-740, 1992) in which the (v_{3}) vibrational term value was determined to be (3131.37 \mathrm{~cm}^{-1}). In our ab initio calculation we obtain (3114.1 \mathrm{~cm}^{-1}) for this term value. Thus, to make our predictions as reliable as possible, we calculate the rovibronic energies after adjusting one parameter in the potential so that we fit the observed value of the (\nu_{3}) vibrational term value. Using this slightly adjusted (a b) initio potential we predict a large number of rovibronic term values in the (\tilde{X}) and (\tilde{A}) states that have not yet been spectroscopically characterized, and we compare our results for the line positions and spin splittings in the (\nu_{3}) band with the observations. 1995 Academic Press. Inc.