Abstract A mathematical model is a tool used to obtain information about the behavior of a system. The mathematical models can be used to have a preliminary knowledge about the functioning of a system, reducing the product costs and improving the performance. In this study, the modified models of several existing models, Gompertz, Orskov, Logistic and Monomolecular were developed that basically divided gas production into two fractions, one arising from rapidly fermentable feed components and the other from slowly fermentable feed components. The aim of this study is related to how originally used models are converted into two fractions mentioned above. For this purpose, all steps of the transformations were given step by step.