Chemical equilibrium models for simulation of thermochemical processes such as feedstock gasification developed so far either focus on the determination of several producer gas compositions or include a parametric study of feedstock depending parameters such as relative fuel/air ratio (F-rg) and moisture content of biomass (h). This article presents a thermochemical process model based on the H-2/C ratio and relative oxygen content (ROC). Therefore, all oxygen-, hydrogen-, and carbon-containing components inside a gasifier are considered, and the model enables the prediction of feedstock behavior as well as an optimization of the feedstock gasification. Furthermore, producer gas compositions are determinable in a unique way as well as the achievable producer gas compositions deriving from several feedstocks combined with several gasifying agents (O-2 and H2O). The calculated results show that the area of achievable producer gas compositions (AAPGC) derived from hard coal clearly exceeds the AAPGC derived from biomass. Whereas the AAPGC derived from lignite only exceeds the AAPGC derived from biomass for low H-2/C ratios. The LHV of the producer gases decreases with increasing the H-2/C ratio and ROC values. However, the resulting gas flow for the steam gasification is clearly higher compared to that of the oxygen gasification.