The apparent rate of CO release during pyrolysis, from bark and Medium Density Fibreboard (MDF) particles was measured. The method used was a pre heated wire grid technique. Experiments were performed under ambient nitrogen and at atmospheric pressure conditions over a temperature range 620940 K. The temperature history of each pyrolysis experiment has been recorded. The CO release was monitored using a tunable diode laser set to a single infrared absorption line of 2082 cm-1. The release followed a typical growth curve in time to a steady state level. Curves were fitted using a simple first order kinetic analysis and Arrhenius parameters were extracted. Both uncertainties in applied temperature and release rate were taken into account. The calculated apparent activation energies Ea were 64 ± 6 kJ mol-1 for bark and 71 ± 6 kJ mol-1 for MDF, uncertainties are determined on a very conservative basis. A single apparent rate seems to be a fair description although two time scales can be identified.