Spinodal has been derived from the chemical potential of the Flory equation of state theory. Also given are the temperature and pressure dependences of the equation of state parameters. Calculations show that the Flory theory describes the compatibility behaviour of polymer mixtures as well as McMaster's modified treatment did.It is pointed out that previous work exaggerated the influence of molecular weight and the equation of state parameters upon phase diagram and predicted a narrow concentration region for phase separation on the assumption that X12=O and Q12=O

where X12 and Q12 are interaction energy and entropy parameters

respectively. For practical systems X12=O and Q12=O

the influence of these factors upon phase diagram and the shape of phase diagram will be predicated to be reasonable as compared with experi-ment.