-He Mixtures.*

An accurate compositionally dependent ρ-P equation of state (EOS) for H2-He is necessary for constraining and validating internal structure and evolutionary models of gas giant planetary interiors. A high accuracy (within 1%) EOS for this system is also required for precompression experiments on these components. The EOS was determined from sound velocity measurements obtained with Brillouin scattering on 9:1 and 1:1 H2:He mixtures at 300 K with pressures up to 5.4 GPa. The ρ-P relations are accurate over the pressure range studied and indicate ideal mixing at both compositions within experimental error. Related physical quantities and methods for the calculation of the equation of state are discussed.