In this work, we report new experimental data on the molar heat capacities of two chemically stable diamines, which are potential absorbents for CO2 capture. They are N,N,N′,N′-tetramethylethylenediamine (TMEDA) and N,N,N′,N′-tetramethyl-1,3-propanediamine (TMPDA). The molar heat capacities of their corresponding aqueous binary and ternary (with piperazine) mixtures were also measured. Measurements were performed at temperatures from 303.15 to 353.15K by heat flow differential scanning calorimetry. The excess molar heat capacities of the binary mixtures were determined and a Redlich–Kister-type equation was applied to correlate the data with temperature and composition. The molar heat capacities of the ternary mixtures were represented as a function of temperature and amine concentration using a modified form of the Sohnel and Novotny equation. The applied correlations satisfactorily represented the heat capacity data as suggested by the low values of the average absolute deviation of 0.2%, 0.4%, and 0.1% for the pure, binary, and ternary systems, respectively.