Fourier transform microwave spectroscopy has been used to investigate vibrational excitation and relaxation of diatomic molecules produced by an electric discharge in the throat of a supersonic nozzle. Rotational transitions of SO, SiO, and SiS, in vibrational states up to v=33 for (SO)-S-32-O-16, v=45 for (SiO)-Si-28-O-16, and v=51 for (SiS)-Si-28-S-32 in their ground electronic states have been detected. The isotopic species (SO)-S-33-O-16, (SO)-S-34-O-16, (SiO)-Si-29-O-16, (SiO)-Si-28-O-18, (SiS)-Si-29-S-32, and (SiS)-Si-28-S-34 have also been observed in highly excited vibrational states. Microwave transitions include up to v=22 for the second lowest excited electronic state b (1)Sigma(+) of SO (similar to10 510 cm-1 above ground) have also been detected. Effective vibrational temperatures have been derived for each species, and a general model is proposed to qualitatively explain the observations. Vibrational excitation is caused by inelastic collisions with the hot electrons produced in the discharge. The subsequent vibrational populations are largely determined by vibration-vibration energy transfer via molecule-molecule binary collisions. Two regions can be inferred from the data: one characterized by a temperature of around 1000 K and a second region with a temperature of several thousand degrees Kelvin. Improved Dunham coefficients and correction terms for the breakdown of the Born-Oppenheimer approximation have been determined for b (1)Sigma(+) SO, X (1)Sigma(+) SiO, and X (1)Sigma(+) SiS. Nuclear spin-rotation hyperfine structure for the Si-29 isotopic species of SiO and SiS has been observed in all highly excited vibrational states. (C) 2003 American Institute of Physics.