Spectral pyrometry was used to determine the dynamics of microwave heating in air of samples (3 g samples) of cotton fibers pretreated with 2, 5, 7, and 10 % aqueous H3PO4 solution. Microwave heating was carried out in an open quartz reactor. The processing of the recorded radiation spectra and the calculation of the temperature were carried out using the “Spectral pyrometry” program. It has been established that the graph of the change in the temperature of the sample during heating has three characteristic regions — low-temperature, the region of unstable temperature, and the region of temperature stabilization. In the low-temperature region, free moisture is removed from the sample. The region of unstable temperature is associated with the process of cellulose combustion, which begins after the end of moisture evaporation at a temperature above 600 K. Under the influence of microwave radiation, all chemical processes intensify in the sample. Video filming during microwave heating showed that in the process of irradiation, sinuous slit-like cavities are formed on the surface of the sample, inside which electrical discharges and ignitions form “hot spots”, from which heat spreads to the entire volume of the sample and additionally heats it, which leads to significant temperature fluctuations. The combustion process continues until all the material is carbonized. After that, a relative stabilization of the temperature occurs, the value of which lies in the range of 1000 – 1200 K. It has been established that the higher the concentration of the H3PO4 solution used for pretreatment of the samples, the lower their temperature in the given area. It has been shown that the carbonization process begins in 70 – 75 s from the start of heating in the local areas of the sample. The results of comparative studies of the structure and elemental composition of samples before and after microwave exposure are presented. It is shown that after 180 s of heating, the fibers become significantly thinner, while maintaining integrity along the length. Carbonized fibers contain a significant amount of phosphorus, the concentration of which is maximum in the outer layers of the sample and is 15 – 20 %. The total weight loss of the samples depends on the concentration of the H3PO4 solution used to pre-impregnate the samples and ranges from 94 % for 2 % H3PO4 to 82 % for 10 % H3PO4.