Temperature dependences of the isoplethic enthalpy of heating DELTAH(isop,heat) for compositions in the CaO . SiO2(CS)-2CaO . Al1O3 . SiO2(C2AS), CaO . SiO2-CaO . Al2O3 . 2SiO2(CAS2) and 2CaO . Al2O3 . SiO2-CaO . Al2O3 . 2SiO2 systems were determined. These quantities were determined using the non-isothermal Hess law, relative enthalpies of individual phases and phase diagrams. The resulting plots allow the direct evaluation of DELTAH(isop,heat) values between any two temperatures for a chosen composition. In the CS-C2AS and CS-CAS2 systems, a narrow temperature range around 1800 K is present in which all mixtures with 0-0.4 CS have the same value of DELTAH(isop,heat) of about 450 kJ mol-1, but with different proportions of the coexisting phases. Within the single-phase melt region, the DELTAH(isop,heat) value of all three systems varies linearly with temperature.