[en] The vibronic coupling between ²Π and ²Σ electronic states of a linear molecule is considered with the inclusion of the spin-orbit coupling of the ²Π electronic state, employing the microscopic (Breit-Pauli) spin-orbit coupling operator in the single-electron approximation. The 6 x 6 Hamiltonian matrix in a diabatic spin-electronic basis is derived by an expansion of the molecular Hamiltonian in powers of the bending amplitude up to second-order. The symmetry properties of the Hamiltonian are analyzed. It is pointed out that there exist zeroth-, first-, and second-order Σ-Π vibronic-coupling terms of spin-orbit origin, which are absent when the usual phenomenological form of the spin-orbit coupling operator is used instead of its microscopic form. The influence of the Σ-Π and spin-orbit vibronic-coupling terms on the adiabatic potential energy curves as well as on the vibronic spectra is analyzed for selected models. It is demonstrated that the interplay of strong Σ-Π vibronic-coupling and strong spin-orbit splitting of the Π state can result in unexpectedly complex vibronic spectra