The automation of existing in vitro gas production methods (Beuvink et al., 1992) and the development of new ones (Theodorou et al., 1994) have created a need for suitable mathematical models to describe and interpret cumulative gas production profiles. Ideally a function is required which is capable of modelling a range of shapes with no inflexion point and also a range of sigmoidal shapes in which the inflexion point is variable. Several models have been proposed to describe gas production profiles (e.g. Blümmel and Ørskov, 1993; Beuvink and Kogut, 1993; Schofield et al ., 1994), sometimes blending empiricism with a more mechanistic view based on a compartmental scheme. The primary objectives of this paper are to present a unifying analysis of this area, pointing out compartmental interpretations of some of the candidate models and to link the gas production technique to animal performance by determining the extent of ruminal degradation for each model.