The stability of a semicontinuous mixture of the type continuous ensemble + discrete component is treated by continuous thermodynamics based on an equation of state. Its parameters are assumed to depend on the molar distribution function of the continuous ensemble by only the number average of the identification variable (molar mass, C-number, etc.) of the species. Equations to calculate the spinodal and the critical point are derived. For demonstration, the method is applied to a semicontinuous n-alkane mixture with methane as discrete component. Using the Soave-Redlich-Kwong equation of state spinodal and critical point are calculated describing the polydispersity by a Gamma distribution.