Monitoring of free gas embedded in scattering media, such as wood, fruits, and synthetic materials, is demonstrated by use of diode laser spectroscopy combined with sensitive modulation techniques. Gas detection is made possible by the contrast of the narrow absorptive feature of the free-gas molecules with the slow wavelength dependence of the absorption and scattering cross sections in solids and liquids. An absorption sensitivity of 2.5 x 10(-4), corresponding to a 1.25-mm air column, is demonstrated by measurements of dispersed molecular oxygen. These techniques open up new possibilities for characterization and diagnostics, including internal gas pressure and gas-exchange assessment, in organic and synthetic materials.