The effect of composting on the production of syngas, a mixture of hydrogen (H-2) and carbon monoxide (CO) with an explicit H-2/CO = 2 or H-2/CO = 3 during pyrolysis of six non-hybrid grasses (NHG) and three hybrid grasses (HG) was investigated. The pyrolysis experiments were conducted in a fixed bed reactor to a final temperature of 700 degrees C. The experimental results obtained in this study demonstrated that composting changed the composition of biomass, significantly reduced the content of carbon dioxide (CO2) and made H-2 and CO the primary constituents of pyrolysis gas. A significant increase in syngas concentration and a higher heating value (HHV), followed by a decrease in the temperature at which a specific H-2/CO was reached, was observed due to composting as well. The change was associated with thermal decomposition of biochemical components, formation of humic substances, and secondary reactions of char. (C) 2015 Elsevier Ltd. All rights reserved.