The X-ray Computed Tomography (X-ray CT) represents a progressive non-destructive method of analysing the inner structure of materials. The method was used for monitoring changes in inner structure of concrete samples of different composition before and after their exposure to various thermal loads. Eight types of concrete samples were prepared which differed by cement and aggregate types. We intentionally used such composition of concrete mixtures which increased their resistance to high temperatures. The inner structure of samples was analysed using the micro X-ray CT before the thermal loading and after exposure of samples to temperatures of 600 °C or 900 °C. Damages and character of changes in the inner structure caused by high temperatures were influenced by the character of cement and aggregate we used. The analysis of obtained results is presented in the paper. ARTICLE INFO