The Rayleigh–Brillouin spectrum of light scattered from H2, D2, and HD has been measured at densities ranging from 1/2 to 104 amagat, all at room temperature. The apparatus consisted of a 20 mW He–Ne laser, a high pressure cell for 90° scattering, a pressure scanned Fabry–Perot interferometer, and photon counting electronics. A discrepancy is noted between the observed spectra and various theoretical spectra. It is suggested that the discrepancy arises from an omission in the theories of coupling between relaxation of the rotational states and thermal conduction.