The energy region just above the first dissociation limit in SO is investigated by a combination of jet cooled 1+1 multiphoton ionization spectroscopy and ab initio calculations. Two electronic states, the C (3)Pi and d (1)Pi states, are found to give sharp, rotationally resolved spectra, and a complete analysis of these is presented for the first time. The potential energy curve for the d (1)Pi is found to be unusual, with only four bound levels and a maximum that does not correspond to a conventional avoided crossing. Our assignments are supported by ab initio calculations, which also predict the interactions between electronic states from spin-orbit coupling. Experimental evidence for these, in the form of perturbations in the observed spectra and predissociation is discussed. Based on our ab initio calculations we also reassign some previous observations to the e (1)Pi state. (C) 2000 American Institute of Physics. [S0021-9606(00)01514-2].