Biomass pyrolysis is usually characterized on the basis of temperature and heating rate. Unfortunately, these parameters are badly defined in processing reactors as well as in laboratory devices. From the results of simplified models, the present paper points out the significant mistakes that can be made when assuming that the actual temperature and heating rate of reacting biomass particles are the same as those of the external heating medium. The difficulties in defining these two parameters are underlined in both cases of a heat source temperature supposed to be constant or to increase with time.