Since first proposed by Johnson et al. (1994), the sulphur hexafluoride (SF₆) tracer technique has been used to quantify ruminal methane emissions from both cattle and sheep. To make the SF₆ technique more robust, permeation tubes with higher release rates have been developed (Hegarty and Woodgate 2003). Although SF₆ is thought to be an inert and non-toxic marker gas, the higher release rates from the new capsules could invalidate this method of determining methane production if there is any effect on microbial metabolism. The intra-ruminal concentration of SF₆ after inserting capsules releasing 100-200 mg/d was from 1-35μl/l(Goopy et al. 2003). A study was made to determine whether SF₆ had any effect on microbial VFA production in the range of 1-100 μl/l.