The in vitro gas production technique has been frequently used to assess biological values of feeds based on their pattern of accumulated gas during incubation with rumen fluid under anaerobic conditions. After data is collected, kinetic parameters that accurately describe the pattern of fermentation can be obtained. Several models have been described and used to fit in vitro gas production data to nonlinear functions (López et al., 1999). The GasFit System was developed to evaluate several nonlinear models in fitting gas production data.