Multiple infrared photon absorption is a quite general process which molecules can undergo when placed in a high flux of infrared energy, such as the focussed beam of a CO2 laser. Among the important consequences of this kind of absorption are isotopically selective molecular dissociation and stimulation of specific chemical reactions. In order to understand how this process works, we must be able to follow the evolution of the molecules through their internal states, populated by photon absorption. Double resonance spectroscopy is the method of a choice for getting at this information. A system pumped by CO2 laser radiation can be examined with a tunable laser probe beam, such as that from a lead-salt diode laser. From such an experiment, we can directly observe Rabi modulation of the absorption lines, determine elementary state-to-state relaxation pathsways, and locate higher excited vibrational states. Systems currently under investigation include SF6 and vinyl chloride. In suitable cases, the probe beam can be a tunable visible or u.v. source, such as a dye laser. Fluorescence spectroscopy can then be used to monitor the transient absorptions produced by multiple-photon excitation. Among the systems which can be examined in this way are biacetyl and glyoxal.