The Edmond–Ogston model is a thermodynamic model proposed by Elizabeth Edmond and Alexander George Ogston in 1968 to describe phase separation of two-component polymer mixtures in a common solvent. At the core of the model is an expression for the Helmholtz free energy 
that takes into account terms in the concentration of the polymers up to second order, and needs three virial coefficients  and  as input. Here  is the molar concentration of polymer ,  is the universal gas constant,  is the absolute temperature,  is the system volume. It is possible to obtain explicit solutions for the coordinates of the critical point 
where  represents the slope of the binodal and spinodal in the critical point. Its value can be obtained by solving a third order polynomial in ,
which can be done analytically using Cardano's method and choosing the solution for which both  and  are positive. 
The spinodal can be expressed analytically too, and the Lambert W function has a central role to express the coordinates of binodal and tie-lines.
The model is closely related to the Flory–Huggins model.