Miscibility behaviour was investigated for the blends of homopolymer A with the random copolymer consisting of two components B and C, where the A homopolymer is miscible with homopolymer B but immiscible with homopolymer C. The copolymer employed was poly(methyl methacrylate-stat-styrene) (MMA.S), and the homopolymers were polystyrene (PS), poly(vinyl methyl ether) (PVME), and poly(ethylene oxide) (PEG) or poly(ethylene glycol) (PEG), where PS is immiscible with MMA homopolymer, PVME is miscible with S but immiscible with MMA, and PEO or PEG is miscible with MMA but immiscible with S. UCST-type miscibility was observed for the PEO/MMA.S blends though the homopolymer blends PEO/PMMA had been reported to be of LCST-type. UCST-type miscibility was found for PS/MMA.S as well. On the other hand, PVME/MMA.S showed LCST-type miscibility with miscibility window-like behaviour. Such contrasting miscibility behaviour, i.e. appearances of UCST and LCST, for a series of MMA.S copolymer blends was discussed on the basis of the Flory-Patterson free volume theory. As a result, it was suggested that contribution of the free volume term decreases for PEO/MMA.S (UCST type), increases for PVME/MMA.S (LCST type) and is little changed for PS/MMA.S (UCST type), compared with that for the respective miscible component pairs, PEO/MMA, PVME/S and PS/S. Furthermore, the Flory-Huggins chi parameters between different components were estimated as a temperature-dependent function from dependence of miscibility on the copolymer composition in these blends. In this estimation, the chi parameters determined for PVME/PS and PEO/PMMA by Han et al. and Ito et al., respectively, using neutron scattering technique were used as a standard. The miscible/immiscible boundaries drawn using the X parameters obtained reproduced well the experimental results of the dependence of miscibility on the molecular weight as well as on the copolymer composition. Thus, it was shown that the X parameters for immiscible pairs such as PS/PMMA, PEO/PS and PVME/PMMA can be evaluated by use of the blend type A/B.C dealt with here.