To understand upper critical solution temperature-type (UCST-type) phase behavior of polycarbonate (PC) blends with poly(ethylene-co-1,4-dimethyl cyclohexane terephthalate) (PECT) co-polyesters, thermodynamic analysis for the miscibility of these blends was performed with an equation-of-state theory. The bare interaction energy for each PC/PECT pair calculated from the phase homogenization temperatures of PC/PECT blends using the lattice-fluid theory was positive and increased with the 1,4-dimethyl cyclohexane (CHDM) content in PECT, whereas the phase homogenization temperature of PC/PECT blends observed in the previous study seemed to decrease with CHDM content. It was revealed that the observed decrease in blend phase homogenization temperature with CHDM content in PECT originated from equation-of-state effects, rather than from energetic effects.