The paraffin wax (PW) utilized as a phase change material is promising for thermal energy storage systems. However, the application of this material is limited by itslow thermal conductivity. The fullerene C60 adding in PW can help to eliminate this problem.Therefore, the system PW/C60 is promising for studying its properties. The results of an experimental study of the refractive index of solutions of fullerene C60 in PW of 52...54 °C melting point at the ranges of С60content of 0...0.052 wt. %and temperatures of 41…65 °С are presented in the study. The complex character of the concentration dependence of the refractive index on isotherms for the studied thermodynamic systems has been registeredboth in liquid and solid phases.Data on the effect of the C60 content in PW on the temperatures of the start and finish of its phase transition is presented. At C60 content in PW of 0…0.01 wt. %  the temperatures of the start and finish of the phase transition decreased, at C60 content of 0.01…0.04 wt. %  they increased more than the pure PW temperatures and at C60 content more than 0.04 wt. % they decreased again. The obtained effects of a decrease and increase in the refractive index on the isotherms and temperatures of start and finish of PWcrystallizationcan be explained by structural transformations in PW caused by C60 presence. In the authors' opinion, the reason for the extreme behavior of concentration dependence of the refractive index of objects of study is the effect of C60 on the density fluctuations and the quasi-crystalline structure of the liquid and solid phases.These structural transformations in PW cause a similar change inthe concentration dependencies of the temperatures of the start and finish of the phase transition of objects of study.