In this paper, we study vibrational relaxation of CO molecules with excited electronic states. We consider three electronic terms and account for VV exchanges of vibrational energy within each electronic term, VT transitions of vibrational energy into a translational one, and VE exchange of vibrational energy between electronic terms. The initial vibrational state of the gas is strongly nonequilibrium. The effect of VE exchange on the vibrational relaxation of CO molecules is estimated for different kinds of initial vibrational distributions, in particular, the Treanor and Gordiets ones generalized for gases with electronically excited states. The set of equations of state-to-state vibrational kinetics, together with the gas dynamic equations, is solved numerically in the zero-order approximation of the Chapman–Enskog method for the case of spatially homogeneous relaxation. The following results are obtained: neglecting VE exchanges leads to an incorrect assessment of the number density for each electronic level; however, the error is small for the ground electronic state. It is shown that VE exchanges qualitatively affect the time dependence of the vibrational temperature.