The ground and next excited (1)A(1) potential energy surfaces of the valence-isoelectronic molecules O-3, S-3, SO2, and S2O have been studied using full-valence-space-multiconfiguration self-consistent-field calculations. An open and a ring minimum were found for all species. In each case, interconversion between the C-2v open and closed isomers proceeds via a C-2v transition state which lies on a very sharp ridge separating the basins surrounding the minima. In the immediate vicinity of these transition states, the upper state for each species is found to have a minimum, also in C-2v. Furthermore, not far away on the C-2v ridges, the 1 (1)A(1) and 2 (1)A(1) surfaces intersect conically for all molecules. (C) 1997 American Institute of Physics.