The study of adsorption on opaque powdered materials using transmission IR spectroscopy is described. The method involves diluting the optically dense adsorbent powder in solid KBr and then pressing the mixture into a tungsten grid. These thousands of tiny self-supporting samples make intimate contact with the grid array, allowing efficient thermal control of the sample temperature from 100 K upward. It has been found that gas adsorption may be studied on such samples by diffusion of the gas through the KBr matrix to the adsorption sites on the powder sample. Examples of the adsorption of small molecules, CO and CO2, on carbon surfaces and larger molecules, dimethylmethyl phosphonate and 2-chloroethylethyl sulfide, on a resin and an Al2O3 surface, respectively, are given, where KBr dilution opens up spectral regions that are not accessible in pure samples of the adsorbent. Interactional effects between the KBr support and the OH groups on Al2O3 are observed. A unique six-sample method for comparison of the adsorbent/KBr and pure KBr samples under identical conditions of gas pressure and temperature is described.