Equilibrium properties of hydrogen-helium mixtures under conditions similar to the interior of giant gas planets are studied by means of first principle density functional molecular dynamics simulations. We investigate the molecular and atomic fluid phase of hydrogen with and without the presence of helium for densities between ρ = 0.19 g cm and ρ = 0.66 g cm and temperatures from T = 500K to T = 8000K. Helium has a crucial influence on the ionic and electronic structure of the liquid. Hydrogen molecule bonds are shortened as well as strengthened which leads to more stable hydrogen molecules compared to pure hydrogen for the same thermodynamic conditions. The ab initio treatment of the mixture enables us to investigate the validity of the widely used linear mixing approximation. We find deviations of up to 8% in energy and volume from linear mixing at constant pressure in the region of molecular dissociation.