The basic feature of phase equilibrium in mixtures or compounds of two or more chemical elements is their non-congruence (or `inconguence'), i.e. ability to vary chemical composition (`stoichiometry') of coexisting phases with no violation of total stoichiometry of two-phase system. Non-congruence leads to substantial change in properties of inter-phase boundaries and critical point(s) in comparison with standard phase transitions in ordinary substances. The features and parameters of non-congruent evaporation in high-temperature uranium-oxygen plasma have been studied thoroughly in frames of nuclear safety problem [1, 2]. Non-congruence is general rule. Any phase transition in equilibrium system of two or more chemical elements must be non-congruent. Non-congruence of hypothetical phase transitions in interior of giant planets and brown dwarfs is under discussion. Main points of discussion are so-called `plasma' phase transition (PPT) (for example [3]) and `dissociative' phase transitions (DPT) [4]. Present situation with theoretical and experimental predictions for PPT and DPT in pure hydrogen and helium is discussed as well as the features of their combination in H2/He mixture in frames of "additivity" approximation. Non-congruence of PPT was estimated [5] for PPT variant of Saumon & Chabrier [4], which is widely used in astrophysical applications. The magnitude of the effect proved to be noticeable and its sigh is in agreement with really observed lowering of helium abundance in atmosphere of Jupiter and Saturn. This approves providing of full-size calculation of non-congruence for any theoretically predicted phase transitions (ionization and dissociation driven) in H2/He mixture of astrophysical objects.

REFERENCES
1. Iosilevskiy I., Gryaznov V., et al. Contrib. Plasma Phys. 43, N 5-6, 316 (2003).
2. Ronchi C., Iosilevskiy I., Yakub E. // Equation of State of Uranium Dioxide / Springer, Berlin, 2004, 366 pp.
3. Saumon D., Chabrier G., Van Horn H.M., Astrophys. J. (Suppl) 99, 713 (1995).
4. Scandolo S., Proc. Nat. Ac. Sci. 100, 3051 (2003); Bonev S., Militzer B., Galli G., Phys. Rev. B. 69, 014101 (2004).
5. Ukrainets A. Iosilevskiy I. in Physics of Substance under Extreme Conditions /Ed. V. Fortov, IPCP RAS, Chernogolovka, 2005, p.181 (in Russian).

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