Complex interactions occur between the many competing and sequential chemical reactions during the pyrolysis of cellulose, making the prediction of the pyrolysis products relatively difficult. The purpose of this paper is to present cellulose pyrolysis as a comprehensible interaction of time, temperature and pressure. Appropriate kinetic data for seven first-order global reactions for the pyrolysis of cellulose were found in the literature. A mathematical model was developed, in which the seven reactions occurred simultaneously so long as the feedstock for the particular reaction existed. The seven differential equations representing the reaction rates were numerically integrated simultaneously to obtain the products of pyrolysis as a function of time, temperature, heating rate and pressure. This program was used to predict many results and trends observed in both slow and fast pyrolysis: very high yields of condensible vapors (primary oils) were predicted under high heating rates to modest final temperatures; high char yields were predicted for slow heating rates at low temperatures; high gas yields were predicted for fast pyrolysis at high temperatures.