One mechanism for powering a rocket engine with laser energy is the absorption of the latter by a molecular seed in the low molecular weight working fluid, hydrogen. Of interest, therefore, is the knowledge of the temperature dependence of the coupling molecule's absorption coefficient at the laser wavelength while immersed in the working fluid. To obtain the information at 10.6 /im wavelength for H2O, D2O, and NH3 in H2, measurements were carried out using a Mach Zehnder interferometer with a CO2 laser light source. Measurements at temperatures as high as 6000 K were obtained using a plasma, sustained in the mixture by a second focused high power cw CO2 laser beam, as a heat source. In addition, measurements at temperatures close to 4000 K were obtained without the plasma simply by focusing the high power beam into the mixture. H2O and D2O exhibited absorptivities as high as 1 cm ~ *, more than an order of magnitude larger than expected from prior analysis, and NH3 proved to be surprisingly stable at high temperatures, thereby enhancing the potential of these three molecules as couplers for the laser powered rocket.