Stability theory for fluid mixtures is used to calculate the spinodal and the critical locus for a system containing one discrete light component and one continuously distributed heavy "component". For reduction to practice, the criteria developed here are suitable for molecular-thermodynamic models where the model parameters depend on the distribution variable that characterizes the continuous "component". To illustrate, the Soave-Redlich-Kwong (SRK) equation of state is used to calculate critical properties of alkane mixtures where methane (C-1) is the discrete component and C-2-C-16 comprise a continuous "component". Calculated results show that, while systems containing methane and primarily small paraffins show a Type I critical locus, those containing methane and appreciable amounts of large paraffins show a Type V critical locus. These results are consistent with experimental critical-locus data for C-1-C-3 (Type I) and those for C-1-C-6 (Type V).