Glass transition parameters can be used to study the miscibility, or lack of it, in polymer-modified asphalt binders. In this study, the contribution of thermodynamics of mixing to glass transition was quantitatively assessed in a differential scanning calorimeter for four asphalt binders partially and fully replaced by an epoxy system. The values of heat capacity (C_p) and, subsequently the glass transition temperature (T_g) of all binders were determined to quantify the miscibility based on the entropic changes. Emphasis was also given to examining the enthalpy of mixing as a function of epoxy system composition during curing to ensure that these binders were completely crosslinked for further analyses. In all cases, the positive deviations of the measured T_g of epoxy-modified asphalt binders (T_g,mix) obtained from the ideal mixing rule led to negative values of the entropy of mixing (ΔS_mix^c), dictating the presence of internal repulsive forces between the asphalt and epoxy components. Softer binders were associated with binders of low deviations of T_g,mix values from the ideal mixing rule. Lastly, the partial replacement of asphalt binders by the epoxy system increased the T_g and decreased the amount of ΔS_mix^c, and such performance imposes the formation of immiscible products.