Multiconfigurational second-order perturbation theory has been employed to calculate two-dimensional potential energy surfaces for the lowest low-lying singlet electronic states of CH2BrCl as a function of the two carbonhalogen bonds. The photochemistry of the system is controlled by a nonadiabatic crossing occurring between the (A) over tilde and (B) over tilde bands, attributed to the b(1)A' and c(1)A' states, which are found almost degenerate and forming a near-degeneracy line of almost equidistant C-Br and C-Cl bonds. A crossing point in the near-degeneracy line is identified as a conical intersection in this reduced two-dimensional space. The positions of the conical intersection located at CASSCF, single-state ( SS)-CASPT2, and multistate ( MS)-CASPT2 levels of theory are compared, also paying attention to the nonorthogonality problem of perturbative approaches. To validate the presence of the conical intersection versus an avoided crossing, the geometrical phase effect has been checked using the multiconfigurational MS-CASPT2 wave function.