By applying the principles of microwave Stark modulated spectroscopy to continuous supersonic beams a new technique for high-resolution rotational spectroscopy has been developed. The capabilities of the new technique have been demonstrated through investigations of the spin-rotation splittings in O13CS and HCCCN. Simultaneously the method allowed a diagnostic of supersonic molecular beams. Relative beam intensities, supersonic flow velocities and translational temperatures of pure and seeded beams were determined from spectroscopic measurements of signal intensities, Doppler shifts and line widths, respectively. Microwave-microwave double-resonance experiments established deviations from the Boltzmann populations of rotational energy levels in supersonic molecular beams.