Ketene is formed by thermal elimination of H2O from acetic acid at temperatures above 923 K. In industry, this process is catalyzed by P2O3/P2O5, which is introduced via organic phosphorous compounds. To optimize the ketene formation, a detailed chemical kinetic mechanism for the P2O3/P2O5-catalyzed formation of ketene (CH2=C=O) in the 923-1123 K temperature range has been developed. With the help of Quantum Chemistry methods DFT (at B3LYP/6-311g(d,p)) and ab-initio (G(3)MP(2)B(3)) calculations, a model describing the high temperature CH3C(=O)OH/P2O3/P2O5 system has been developed and combined with the modified mechanism from Mackie and Doolan (1984). The addition of P2O3/P2O5 to acetic acid results in a markedly accelerated ketene formation. The conversion of acetic acid increases with increasing temperature and with higher content of P2O5. The mechanism allows the design of temperature profiles for the ketene synthesis with optimal conversion of acetic acid.