High-sensitivity, microwave-infrared double-resonance measurements can be made in molecular-beam spectrometers employing a single state-focusing device. The key feature of the double-resonance technique is the achievement of large signal modulations of infrared signals using microwave transitions, even in cases where the infrared transition cannot be saturated. A series of measurements is presented that shows that the technique is based on the transition moment and state-focusing properties of dressed molecular states in the presence of a strong microwave held. Using a state-focusing device, the spectroscopic measurements are doubly sensitive to the composition of the dressed states. The technique can be extended to other types of spectroscopy, such as electronic spectroscopy and the spectroscopy of weakly bound complexes. (C) 1997 American Institute of Physics.