Thermal degradation of beech cellulose sample treated with dimethylphosphite, phosphorous oxychloride and diethyl and diethyl ether of 2, 3-epoxypropylphosphonic acid was investigated in the temperature range 40-910°C by pyrolytic mass-spectrometry. CH₄, H₂O, CO, CO₂, phenols and aromatic hydrocarbons were detected in pyrolysis products. On the basis of this method, the yields of volatile pyrolysis products vs. temperature are obtained. Phosphorylation of cellulose shifts the gas formation maximum by 60-120°C; the composition of gaseous mixture becoming more simple.