By the introduction of new gas production equipment, using sensitive pressure transducers, producing highly accurate gas production profiles, new mathematical models were developed to describe the gas production profiles accurately as existing models did not fit the profiles satisfactorily. It has been shown previously that gas production profiles can be described mathematically by a three-phasic model. This paper shows that the different phases in the gas production profiles were caused by fermentation of the soluble fraction and the non-soluble fraction and by turnover of the microbial population. Because the gas production profiles can be divided into different phases it is possible to compare the results with the nylon bag technique and estimating the degradation rate of the soluble faction. Because the major fraction of the gas production in the blank is caused by microbial turnover, which occurs in an earlier stage of incubation in the blank than in the samples, it is discussed whether the subtraction of the blank gas production should better not be done.