The coupling of an internal relaxation to both the chemical reaction and diffusion processes is investigated for a fluid mixture. A microscopic approach beginning with a generalized Langevin equation is used to derive relaxation equations for the concentration fluctuations of the species in specified internal states as well as total concentration fluctuations. These equations are used to calculate the matrix of concentration correlation functions which are then related to the intensity of the light scattered from the fluid. The circumstances under which such a generalized hydrodynamics approach must be used are examined. The possibility of extracting information about both the chemical and internal relaxation processes from a measurement of the width of the polarized component of the Rayleigh line is considered.
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