An upper critical solution temperature (UCST) type of miscibility was observed for high-molecular-weight polymer blends of poly(methyl methacrylate) (PMMA) with poly(n-butyl methacrylate) (PnBMA) and with poly(isobutyl methacrylate) (PiBMA). The blends containing random copolymers consisting of these methacrylate monomer units also showed UCST type miscibility over all copolymer compositions. The segmental interaction parameters chi(MMA/nBMA), chi(MMA/iBMA) and chi(iBMA/nBMA) were estimated from the dependence of the miscibility on the copolymer composition using the Flory-Huggins theory applied to the random copolymer blends. The temperature dependence of chi(MMA/nBMA) and chi(MMA/iBMA) was required to be comparatively strong in the temperature range above chi(crit), while chi(iBMA/nBMA) did not depend so much on the temperature. Quadratic functions of temperature were more appropriate for chi(MMA/nBMA), chi(MMA/iBMA) and chi(iBMA/nBMA) than another type of function that is proportional to 1/T. The absolute values of all chi(i/j) estimated here were very small compared with those for other immiscible polymer blends. This means that the exchange free energy between the different methacrylate components is very small, which is considered to bring UCST-type miscibility to the high-molecular-weight polymer blends studied here.