One of the most important parameters in the selection of composite materials used for the protection of avionics instruments exposed to high temperatures are ablative properties. They constitute the main criterion in determining the composition and thickness of the protective material. In this article, the authors determine the ablative properties of a polymer matrix with carbon reinforcement and check the effect of using the MMT additive (montmorillonite) on the change in the resistance to the impact of a high heat flux. The tested materials play an important role in the defence, aviation and space industry. Ablative materials are the only ones to protect the rear wall surface from an excessive temperature rise while using a thin insulation wall. For the sake of the research, we prepared a series of samples of the composite produced with epoxy resin LH 145, H 147 hardener, carbon fibre mats and the addition of MMT. The prepared samples were tested on a unique stand in laboratory conditions. The findings obtained from the experimental testing after a detailed analysis were tabulated and presented in the form of graphs. The authors determined the ablative loss of mass of the individual samples, compared their internal temperatures, which had been measured with thermocouples, as well as the temperature on the backside of the sample. In addition, in order to complement the experimental studies of determining the temperature rise on the rear surface, the authors used a thermal imaging camera. Besides, they took photos of different layers of the examined structure, which had been exposed, to a heat stream, by means of a scanning microscope.