The glass transition parameters are used to study the miscibility, or lack of it, in polymer modified asphalt binders. In this study, a quantitative assessment of the contribution of thermodynamics of mixing to glass transition was conducted in a differential scanning calorimetry for four asphalt binders modified with an elastomeric epoxy system. Especially, the values of heat capacity (Cp) and subsequently the glass transition temperature (Tg) of all binders were determined to quantify the miscibility based on the entropic changes. Emphasis was also given on examining the enthalpy of mixing as a function of the composition of epoxy asphalt binders during curing to ensure that these binders were completely crosslinked for further analyses. In all cases, the positive deviations of Tg,mix obtained from the ideal mixing rule, or 𝛥𝑇𝑔,𝑚𝑖𝑥, led to negative values of the entropy of mixing (𝛥𝑆𝑚𝑖𝑥𝑐), dictating the presence of internal repulsive forces between the asphalt and epoxy components. The soft in properties and sol type base binders are also associated with epoxy asphalt binders of low 𝛥𝑇𝑔,𝑚𝑖𝑥 values. Overall, the incorporation of the epoxy system in asphalt binders increases the Tg and decreases the amount of 𝛥𝑆𝑚𝑖𝑥𝑐, and such performance imposes the formation of phase-separated binders.