The objective of this study was to investigate effects of biomass constituents (cellulose, hemicellulose and lignin) on biomass thermal decomposition and gas evolution profiles of four biomass materials. Switchgrass, wheat straw, eastern redcedar and dry distilled grains with solubles (DDGS) were selected as the biomass materials. No significant difference was observed in the weight loss profiles of switchgrass, wheat straw and eastern redcedar even though their cellulose, hemicellulose and lignin contents were considerably different. The weight loss kinetic parameters were also not significantly different except for activation energy of the eastern redcedar. However, biomass composition did significantly affect gas evolution profiles. The higher contents of cellulose and hemicellulose in switchgrass and wheat straw may have resulted in their higher CO and CO₂ concentrations as compared to eastern redcedar. On the other hand, higher lignin content in eastern redcedar may have resulted in significantly its high CH₄ concentration.