The in vitro gas production technique is a means of measuring the dynamics of fermentation. It is related to short chain fatty acid (SCFA) production, and so could be used to estimate ATP supply for rumen microorganisms. However, different fermentation patterns produce different amounts of gas. No fermentation gas is associated with the production of propionate, and so an increase in the proportion of propionic: (acetic+butyric) (P:AB) would be associated with a decrease in the volume of gas produced. If the molar proportions of SCFA changed during a fermentation, then this would complicate the interpretation of the gas production profile (GPP). If the GPP, combined with a measure of SCFA concentrations at the end of the incubation, was used to estimate ATP yield during the incubation, then changes in P:AB during the incubation may affect these estimates. The objectives of this experiment were therefore to determine whether P:AB did change during an in vitro incubation, and whether any such change affected the accuracy of the prediction of ATP yield with time.