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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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Anisimov et al. Reply:We thank Bagnuls and Bervillier [1] for emphasizing the difference between regular (noncritical) behavior outside the critical region and mean-field critical behavior, which is characterized by the mean-field (classical) critical exponents within the critical domain. In our paper [2] we definitely refer to the latter case, since we consider the behavior of the susceptibility at t ͑T 2 T c ͒͞T , 10 22 . Bagnuls and Bervillier claim that the nonmonotonic crossover from the classical behavior to the asymptotic scaling behavior within the critical domain is impossible. They are correct if the crossover is controlled by a single crossover parameter. However, the main point of our paper [2] is that the sharp and sometimes nonmonotonic crossover behavior of the susceptibility, observed experimentally within the critical domain for several binary ionic solutions, can be perfectly described by a crossover model that includes two independent crossover parameters.It has been shown by Anisimov et al.[3] that a singleparameter model, based on renormalization-group matching [4], gives a crossover behavior of the free energy similar to those based on the´expansion [5] and on the field theory [6]. In this model, if the systemdependent coupling constant u in the Landau-Ginzburg-Wilson Hamiltonian, reduced by the dimensionless (in units of an average intermolecular distance) microscopic cutoff L, is less than u ‫ء‬ (the universal renormalizationgroup-theory fixed-point value of the coupling constant), i.e., u u͞u ‫ء‬ L , 1, the mean-field behavior is recovered in the limit uL͞k ø 1 and controlled by the Gaussian fixed point at which uL 0. The parameter k is inversely proportional to the correlation length j in such a way that L͞k q D j with q D the actual microscopic cutoff wave number. In simple fluids the cutoff parameter L is of order unity (the characteristic microscopic scale is of the order of a molecular size). This is why the crossover to the classical regime is not completed within the critical domain where j is large, unless there is a special reason for very small u like near tricriticality. If u $ 1, the crossover scale is not defined by this model. For u 1

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