A method for the direct determination of the cellulose and lignin content of biomass has been developed. The method assumes that negligible interactions exist between biomass macrocomponents and it is based on the analysis of thermogravimetric (TG) pyrolysis data. The TG-weighed sum method seems to be reliable and less time consuming than conventional methods for the determination of the cellulose and lignin content. Some limitations of the macrocomponent approach in the description of biomass pyrolysis are also discussed. In particular, hemicellulose pyrolysis data are analyzed. The use of xylans as substitute materials for hemicellulose have been critically examined. The results obtained point out that the widely practiced use of xylans as substitute compounds for the simplified description of hemicellulose pyrolysis can lead to significant errors.