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In variabledensity (VD) turbulent mixing, where verydifferentdensity materials coexist in comparable amounts, the density fluctuations can be an order of magnitude larger than their mean. Density fluctuations are nonnegligible in the inertia terms of the NavierStokes equation which has both quadratic and cubic nonlinearities. Very different mixing rates of different materials give rise to large differential accelerations and some fundamentally new physics that is not seen in constantdensity turbulence. In VD flows material mixing is active in a sense far stronger than that applied in the Boussinesq approximation of buoyantlydriven flows: the mass fraction fluctuations are coupled to each other and to the fluid momentum. Statistical modeling of VD mixing requires accounting for basic constraints that are not important in the smalldensityfluctuation passivescalarmixing approximation: the unitsum of mass fractions, bounded sample space, and the highly skewed nature of the probability densities become essential. We develop the consequences of the mass conservation law for multicomponent mixtures for randomwalk methods in variabledensity turbulence. One consequence of the constraints developed is that the coefficient of the Wiener process must be nonlinear and coupled to the other mass fractions to ensure consistency with mass conservation. Typical Langevintype models for these processes violate these constraints peculiar to VD mixing. Then we derive a transport equation for the joint probability of mass fractions, equivalent to a system of stochastic differential equations, that is consistent with VD mixing in multicomponent turbulence and consistently reduces to passive scalar mixing in constantdensity flows. Host: Shiv K. Sambasivan, T5 6658075 