Ab initio RCCSD(T) and MRCI calculations have been carried out for the stable isomer forms of magnesium monohydroxide, HMgO and monohydro sulfide, HMgS. For both molecular systems, it is found that the electronic ground state is a (2)Pi(i) state with linear/linear Renner-Teller components. At the equilibrium geometry of the ground states, the spin-orbit constants A(SO) have been calculated -123 cm(-1) for HMgO and -312 cm(-1) for HMgS. Along the isomerisation path towards the more stable structures MgOH and MgSH, the (2)A' component of the Renner Teller pair correlates with the ground state and the (2)A" component with an excited state. Using RCCSD(T) potential energy functions and a variational Renner-Teller approach, rovibronic levels of the X(2)Pi state of both systems have been evaluated for J = 1/2 and 3/2. Strong Fermi resonances are found in HMgO, but not in HMgS. The first electronically excited (2)Sigma(divided by) states are calculated (T-e value at RCCSD(T) level) to lie 8041 cm(-1) above the ground state for HMgS and 4935 cm(-1) for HMgO. (C) 2003 Elsevier B.V. All rights reserved.