Five models were studied for goodness of fit to in sacco organic matter disappearance. Except for the first-order model, all the other models showed an accurate and unbiased fit, although in some cases this was achieved by unrealistic parameter estimates or an increase in the number of parameters that need to be estimated. The first-order model showed a biased fit as indicated by a slope which is significantly larger than 1 for the regression between observed and predicted values. Marker passage at the ileum was satisfactorily described by the gamma retention time distribution in 90% of the cases. Only the curve of one sheep, which had a very irregular flow pattern, could not be fitted accurately by this function. The integer form of the gamma distribution of retention time may be used to combine outflow and fermentation in one equation. This equation describes the disappearance of fermentable organic matter, when calculating intake.