Biomass is currently a promising renewable and potentially neutral raw material in relation to global warming. Grey alder (Alnus incana (L.) Moench) wood was chosen for research. Carbonization (slow pyrolysis) of the granulated modified grey alder wood is a standard pre-treatment method for physical activation to obtain valuable high-quality granular activated carbon for cleaning of gaseous emissions. Relative ratios of the main carbonization products of lignocellulosic materials (charcoal, liquid condensate and uncondensable gases) are strongly influenced by the physical and chemical characteristics of the raw materials. It is very important to achieve high charcoal yields and retain the mechanical properties of the obtained carbonized and activated modified wood granules. Recently, extensive research has been carried out in the field of the biomass thermal decomposition behavior during pyrolysis. Hydrothermally treated grey alder wood has not been investigated from this point of view until now. Therefore, the purpose of the investigation is to examine the effect of the hydrothermal treatment time of grey alder wood on the thermal stability of granulated modified grey alder wood carried out in a thermogravimetric/mass spectroscopic (TG-MS) system and a thermogravimetric analysis apparatus (TGA). The behaviour of hydrothermally treated wood during pyrolysis is completely different from that of the initial grey alder wood. By modification of grey alder wood, the cellulose and lignin content increases due to decreasing of the hemicellulose content. A comparison between the hydrothermally treated and initial grey alder wood shows that, at first, decomposition starts at higher temperatures (228 and 214oC, respectively), the maximum of thermal decomposition is reached at a higher temperature (347 and 336oC, respectively), the hemicellulose shoulder in the TGA peak disappears, and the yields of char are more than twice higher (35 and 12%, respectively). The initial and modified (treatment time 1 h and 2 h) grey alder wood was studied also by simultaneous TG-MS in an inert argon atmosphere at a heating rate of 20oC min-1. It is found that gaseous emissions such as H2O, CO2 and CO are the main products formed during pyrolysis for all samples. The differences in the gas formation were found to be due to their differences in the composition of hemicellulose, cellulose and lignin. The major part of those emissions is formed during the thermal decomposition of cellulose at a temperature of 350-450oC. The formation of C2H2+ and C2H3+ shows that crosslinking by recombination and polycondensation reactions (formation of the so-called “pseudolignin”) occurs between cellulose and lignin during pyrolysis. The temperature intervals of the formation of these organic compound fragments are similar to those for the CH4 forming temperature, which means that the highest charcoal yields are obtained from modified wood with the highest cellulose and lignin content comparing to the case of the initial grey alder wood. It is demonstrated by thermogravimetric and mass spectroscopic analysis that the grey alder wood after hydrothermal treatment is thermally more stable than the initial material, and the charcoal yield increases with increasing treatment time. The pellets after carbonization retain their mechanical properties and are an excellent raw material for production of high-density granular activated carbon.