Light scattering and calorimetric techniques were used to investigate the isothermal phase behaviour of a ternary polymer blend consisting of three pairwise miscible homopolymers. These homopolymers, polystyrene (PS), poly(2-chlorostyrene) (P2ClS) and polycyclohexylacrylate (PCHA), are apparently the first known examples of homopolymers that form a closed loop in the ternary isothermal phase diagram. The phase diagram of the ternary blend was determined by light scattering at 433 K, where all three pairs are below their respective lower critical solution temperature (LCST). The miscible compositions exhibit a single glass transition temperature (T(g)) which follow a simple volume additivity relationship. Inside the immiscible closed-loop region, single T(g)s are also observed, but these are shown to be consistent with the presence of two phases. Two binary interaction parameters (for the pairs PS-P2ClS and PS-PCHA) were determined from the locations of LCSTs. The interaction parameter between P2ClS-PCHA was determined by approximating the observed closed-loop region of immiscibility by the Flory-Huggins lattice model. In analogy with systems comprised of a mixture of two polymers and a solvent, the closed-loop immiscibility gap results from an asymmetry in the interaction parameters between the three pairs (a ’DELTA(chi)’ effect). The following ordering was observed : chi(PCHA-P2ClS) < chi(PCHA-PS) almost-equal-to chi(PS-P2ClS). This trend is consistent with the curvatures of the composition-dependent glass transition T(g)(phi) plots.