Single resonance chemical remote sensing, such as Fourier-transform infrared spectroscopy, has limited recognition specificity because of atmospheric pressure broadening. Active interrogation techniques promise much greater chemical recognition that can overcome the limits imposed by atmospheric pressure broadening. Here we introduce infrared - terahertz (IR/THz) double resonance spectroscopy as an active means of chemical remote sensing that retains recognition specificity through rare, molecule-unique coincidences between IR molecular absorption and a line-tunable CO2 excitation laser. The laser-induced double resonance is observed as a modulated THz spectrum monitored by a THz transceiver. As an example, our analysis indicates that a 1 ppm cloud of CH3F 100 m thick can be detected at distances up to 1 km using this technique. © 2010 Copyright SPIE - The International Society for Optical Engineering.