Abstract This paper provides in-depth knowledge about the evolved gas analysis interpretation via newly proposed semi-quantitative approach, arising from thermogravimetric analysis (TGA) – mass spectrometry (MS) coupled measurements, for studying pyrolysis behavior of three kinds of biomass waste materials (spent coffee grounds, beech sawdust and wheat straw). TGA – MS coupling allows accurate correlation between molecular ion peak and fragment peaks to the corresponding mass loss rates from derivative thermogravimetry curves. Within proposed semi-quantitative analysis, MS spectra were interpreted through the comparative analysis of compounds fragments and of the compound itself, where the single atomic mass unit was identified by multiple compounds exhibition. It was shown that by this procedure which involves overlapping multiple curves supervising, the identification of gases in volatiles complex scheme becomes more simplified. By setting up semi-quantitative formulas, easy and reliable calculations of gaseous products yield and syngas energy capacities are possible to achieve. The H2/CO ratio derived from the proposed method for wood waste product (sawdust) is in an excellent agreement with H2/CO ratio for sawdust syngas production, in fuel reactor for biomass gasification and H2 production.