A volume modulation technique is employed to measure the complex compressibility of a gas in reticulated vitreous carbon (RVC). The complex compressibility depends on the geometry-dependent thermal coupling between the gas and the solid walls, and is related to the thermoacoustic function fκ, or equivalently F(λT). By measuring samples of different cross section and length, all external boundary effects are eliminated, so that F(λT) corresponding to bulk RVC is determined. Working down to very low frequencies achieves a wide range of values for the ratio of the thermal penetration depth to the mean pore size. As a test of the concept of “capillary-based porous media,” the results are compared to analytical solutions for F(λT) in circular, parallel plate and pin-array geometries. Measurements made at peak-to-peak gas displacement amplitudes exceeding the pore size in at least half of the sample showed no observable deviation from the small-amplitude results. © 2001 Acoustical Society of America.