We investigate the dissociation equilibrium of H_2 in very cool, helium-rich white dwarf atmospheres. We present the nonideal chemical equilibrium solution for the dissociation of molecular hydrogen in a medium of dense helium. We find that at the photosphere of cool white dwarfs of T_(eff) = 4000 K, the nonideality results in an increase of the mole fraction of molecular hydrogen by up to a factor of ～ 10 compared to the equilibrium value for the ideal gas. This increases the H_2-He collision-induced absorption (CIA) opacity by an order of magnitude and will affect the determination of the abundance of hydrogen in very cool, helium-rich white dwarfs.
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