Photoacoustic signals from opaque thermally thin solids (Ni, Al, and SiC) with a gas backing have an ω−3/2 frequency dependence. This first experimental observation of such a dependence from thin solids is in excellent agreement with the Rosencwaig-Gersho theory when a low-conductivity backing material is considered, but in disagreement with predictions based on the theory of Cesar et al. Surface heat conductance is determined by molecular collisions with the surface and not by blackbody radiative transfer.