This article deals with a remarkable phenomenon in chemical-phase theory, i.e., the limited mutual solubility of gases, which was predicted by van der Waals and first demonstrated by Krichevskii in the nitrogen—ammonia system. The phase behavior in question is characterized by a peculiar course of the critical locus in some systems for which the components have a considerable difference in volatility. Under special conditions such a system showing limited miscibility may also exhibit the barotropic phenomenon, i.e., the equilibrium phases change position in the gravitational field. Using the conditions Temkin derived from van der Waals' applications of Korteweg's fold theory, we could predict that the helium—xenon system probably exhibits gas—gas equilibria. We have confirmed the predicted behavior by visual observations of phase transitions in helium—xenon up to 2000 atm and 65°C.