Stability theory for multicomponent fluid mixtures is used to calculate the spinodal and the critical locus for a multicomponent mixture. Upon using well-defined simplifying assumptions, the criteria developed here are suitable for mixtures whose properties are described by a molecular-thermodynamic model where the Helmholtz energy depends on composition-average model parameters. Such mixtures are typically encountered in systems that contain homologues. Our method is especially useful when the homologue-mixture contains a large number of components. Different from our previous work (Fluid Phase Equilibria, 206, 41-59, 2003), the criteria developed here do not require that the model parameters depend on moments of a characteristic property (e.g. molecular weight) used to characterize the distribution of components. The spinodal lines and the critical points obtained by our method are identical to those obtained from the less-computer-efficient Gibbs traditional method. To illustrate, the Soave-Redlich-Kwong equation of state is used to calculate critical properties of mixtures containing carbon dioxide, hydrogen sulphide, and alkanes. (C) 2009 Elsevier Ltd. All rights reserved.