As found by thermogravimetric and differential thermal analysis and gas chromatography, the thermal oxidative pretreatment of cellulose and cellulose compositions with high-molecular-weight polycondensed aromatic systems of natural origin resulted in a shift of peaks due to intense thermal degradation to the region of lower temperatures and changes in the yields of carbon residues and in the quantitative composition of the main gaseous products of pyrolysis. In the presence of ammonium molybdate, which is a catalyst for cellulose oxidation and carbonization, the effect of the thermal oxidative treatment was even more enhanced. This was reflected in an increase in the low-temperature shifts of intense degradation processes and a considerable increase in the yields of carbon residues of cellulose and cellulose compositions.