Nanofluids are promising heat carriers, which contribute to the overall efficiency of energy systems. The main obstacle to the practical application of nanocoolants based on aqueous propylene glycol solutions is the lack of accurate data on their thermophysical properties. In the paper, experimental study (adiabatic calorimetry method) of the heat capacity and parameters of solid phase – liquid phase transitions of propylene glycol and coolant based on aqueous propylene glycol solution is carried out. Experimental study of the heat capacity of the liquid phase of the coolant based on an aqueous solution of propylene glycol with additives of Al 2 O 3 nanoparticles (up to 2.01 wt. %) in the temperature range of 235...338 K and propylene glycol with additives of Al 2 O 3 nanoparticles (1.03 wt. %) in the temperature range of 268…335 K is performed. The comparison of the temperature dependence of the effective heat capacity of coolants with changes in their internal structure is made. It is shown that adding water to propylene glycol increases the temperature and heat of the solid phase – liquid phase transition (the heat of the propylene glycol phase transition is 37.85 J∙g –1 , propylene glycol/water coolant (54/46 wt. %) – 77.97 J∙g –1 ). It is shown that additives of Al 2 O 3 nanoparticles both in propylene glycol and in the coolant based on an aqueous propylene glycol solution contribute to the reduction of the heat capacity of the liquid. The heat capacity decreases approximately in proportion to the increase in the concentration of nanoparticles. The effect of heat capacity reduction is greater at high temperatures (3.9 % at 265 K and 5.0 % at 325 K for the nanocoolant with an Al 2 O 3 nanoparticle concentration of 2.01 wt. %). The results obtained will improve the design quality of heat exchange equipment using nanocoolants. The results are useful for developing methods for predicting the specific heat of nanofluids