An analysis of experimental data for the susceptibility of fIuids and of liquid mixtures in the critical region has been performed to elucidate the character of the nonasymptotic critical behavior. While fluids and fluid mixtures exhibit an ultimate crossover to Ising-like asymptotic behavior, the effective susceptibility exponent y,tt approaches the universal value y = 1.24 either from above (A) or from below (B). We conclude that simple fluids belong to type B, whereas more complex systems may belong to type A and show a sharper nonmonotonic crossover from mean-field to Ising-like behavior. PACS numbers: 64.60.Fr, 05.70.JkIt has been well established that isotropic fluids belong to the three-dimensional (3D) anisotropic Ising model (equivalent to the 3D lattice gas) universality class.Specifically, the susceptibility~(isothermal compressibility in one-component fluids and osmotic compressibility in liquid mixtures) in zero field in the one-phase region asymptotically close to the critical point behaves as

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Two hydrodynamic relaxation modes associated with mass diffusion and thermal diffusion are present in binary fluids. In near-critical binary fluids a coupling between the two modes results in two characteristic relaxation times, neither of which is associated with pure mass diffusion or pure thermal diffusion. Instead, the relaxation times are inversely proportional to two effective diffusivities D 1 and D 2 , which can be detected experimentally by dynamic light scattering. The physical meaning of D 1 and D 2 changes as one considers states in the vicinity of different points on the critical locus: in the infinite-dilution limit the diffusivity D 1 of the slow mode is associated with the thermal diffusivity and the diffusivity D 2 of the fast mode with the mutual mass diffusion coefficient, while in the ''incompressible'' liquid-mixture limit D 1 is associated with the mass diffusion coefficient and D 2 with the thermal diffusivity. In addition we have determined the intensities ͑amplitudes͒ of these relaxation modes, which can also be measured with light scattering. We discuss the conditions at which a two-exponential decay of the dynamic correlation function can be measured. As an example we consider mixtures of methane and ethane near the vapor-liquid critical line where the two exponential decays indeed have been observed. ͓S1063-651X͑98͒05402-6͔

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