An internal rotor model to fit and simulate rotationally resolved electronic spectra of molecules or complexes with an internal top of C-2v symmetry and a frame of symmetry C-s or C-2v in the semirigid approximation is described. The rotation-internal rotation problem is solved independently for both electronic states by numerical diagonalization of the energy matrices, which are factored according to the symmetry properties of the Hamiltonian. Finally, selection rules and line strength expressions for the simulation of electronic spectra are given.