A procedure is described that uses two spectroscopic techniques, absorption and infrared degenerate four-wave mixing, in tandem (multiplex) to measure the transition dipole moments and absolute concentrations of molecular species in situ. The method is demonstrated by the measurement of the relative transition moments and concentrations of two dissimilar sample gas components, hydrogen chloride and nitrogen dioxide, but is applicable to a wide variety of molecules and, thus, can provide new information for transient molecular species. Further, difficulties in obtaining quantitative information through techniques such as laser-induced fluorescence, coherent antiStokes Raman scattering, and degenerate four-wave mixing spectroscopies can be overcome when a multiplex approach is used.