The phase behaviour of ternary polymer blend of poly(methyl methacrylate) (PMMA), poly(ethylene oxide) (PEO) and poly(hydroxy ether of bisphenol-A) (phenoxy) was investigated. Although all three binary blends are separately miscible, there exists a closed immiscibility loop in the phase diagram of ternary blends. It may result from small differences in the interaction energy of binary systems, the so-called ’Delta(chi) effect’. The value of chi(phenoxy/PMMA), by comparing the theoretically predicted phase diagram with experimental data, was determined to be -0.61.