The phase behaviour of ternary copolymer blends poly(styrene-stat-acrylonitrile) (SAN)(poly(methyl methacrylate-stat-acrylonitrile) (MAN)/poly(N-phenylitaconimide-stat-methyl methacrylate) (PIMMMA) was analysed using Flory-Huggins theory. The system studied consists of the combination of three binary pairs SAN/MAN, SAN/PIMMMA and MAN/PIMMMA, and it has been demonstrated that the combination of these three miscible pairings cannot guarantee the formation of a single-phase ternary blend. Phase-separated regions in ternary space can appear at compositions defined by the miscible windows of the binary pairings, provided that the difference in the interactions between them is large. When the molecular weights of the three polymers are unequal it is suggested that a modified term Delta(chi)’ = chi’(13)-chi’(12)) is used to replace the normal Delta chi, in order to reflect the influence of molecular weight on the asymmetry of the interactions between the different binary polymer pairs. The new term chi(ij)=(chi(ijcrit)-chi(ij)), where chi(ijcrit)> > chi(ip) is the difference between the critical interaction parameter and the corresponding normal interaction parameter of the i-j pair. It will equal the original expression for Delta chi only when the component molecular weights are equal.