Abstract <p>The influence of the content of fibrous basalt on the temperature dependence of the specific volume of composites based on thermoplastic elastomer was investigated. A mechanical mixture of polypropylene with 30 wt % butadiene-styrene copolymer was used as a thermoplastic elastomer. To improve the technological compatibility of the mixed components of the mixture, a compatibilizer was used, which was modified with 1.5 wt % maleic anhydride content. The content of fibrous basalt in the composition of the thermoplastic elastomer varied within the range of 5.0–20 wt %. It was shown that for composites with 5.0–15 wt % fibrous basalt content, the first-order phase transition occurs at a temperature of 159°C. For a sample with 20 wt % fibrous basalt content, the value of this indicator increases to 168°C. Based on the data of dilatometric curves constructed in the temperature range of 20–210°C, a decrease in the specific volume or an increase in the density of composites with an increase in the content of fibrous basalt was recorded. Only at 20 wt % filler content did the specific volume increase in this temperature range. By extrapolating the dilatometric curves to the region of absolute zero (–273°C) for composites with different fiber filler contents, the “occupied” specific volume was found. This made it possible to construct a dependence of the “free” specific volume on temperature. At the temperature of the first-order phase transition, i.e. from a viscous-flowing to a solid state, the mechanism and kinetic regularities of crystallization were studied in Kolmogorov–Avrami coordinates under the condition of continuous formation of nucleation centers. It was established that in filled thermoplastic elastomers the isothermal crystallization process proceeds according to the linear (rod-like) type of growth of crystalline formations.</p>