Large scale Multireference Configuration Interactions (MRCI) and energy consistent relativistic pseudopotential (for the Hg atom) have been used to investigate the electronic structure, stability and spectroscopy of the low lying electronic states of the HgS molecule. The relative position of the two lowest electronic states, X-1 Sigma(+) and a(3)Pi, was found to be very sensitive to the quality of the basis set. Spin-orbit effects were taken into account leading to accurate spectroscopic data useful for the identification of the molecule. T-0 between the lowest components of the two states, X-1 Sigma(+)(0) and a(3)Pi(2), has been evaluated to be 0.142eV (3.5 kcal mol(-1)). Dipole moment functions were calculated for the lowest states; the rather large dipole moment of the X-1 Sigma(+) state makes possible the detection of vibrational transitions with a calculated ! e equal to 364cm(-1). Transitions between the X-1 Sigma(+) and the A(1)Pi states are predicted in the far IR domain with a T-0 = 5794cm(-1). The predissociation of the X-1 Sigma(+) and A(1)Pi states has been analysed and it has been shown that for the X-1 Sigma(+)(0) state only the vibrational levels below v = 11 are stable; higher levels are predissociated by the a(3)Pi(0) state. The effective dissociation energy of the X-1 Sigma(+)(0) state of HgS can thus be estimated to be 0.47eV (6.5 kcal mol(-1)). For the A(1)Pi state, the levels with v > 8 are predissociated by the dissociative b(3)Sigma(-) state.