Two years ago, at the CORESTA Congress in Brighton, the authors presented a paper on the effect of heating rate on the evolution of formaldehyde during cellulose pyrolysis. Since then the work has been expanded to include kinetic data on three other gas phase species: hydroxyacetaldehyde, CO and CO₂. Yield data have been obtained for the condensable tars as a function of heating rate. A method has been developed for solving the rate equations for competing, non-isothermal reactions with exponential temperature profiles. The scope of the work has, thus, been considerably broadened. The expanded study is presented here in two parts. The present paper deals with the determination of rate constants while the second, which follows, is concerned with mechanistic implications.The yields of formaldehyde, CO and hydroxyacetaldehyde all increased with heating rate. The kinetics of formaldehyde and CO evolution could be described by a pair of competing reactions. The kinetics of hydroxyacetaldehyde required a third rate constant indicating that an intermediate is involved. Carbon dioxide evolution declined slightly with heating rate. However, since the decrease was too small to resolve, the kinetics of CO₂ formation were treated as heating rate independent. Kinetic data were not available for tar. Nevertheless it was possible to estimate rate constants from the variation in yields. The rate laws are presented. A comparison of rate constants suggests a mechanism involving competing reactions with the high temperature reaction yielding formaldehyde, hydroxyacetaldehyde and CO while the low temperature reaction leads to tar and CO₂.