The semi-automated Reading Pressure Technique (RPT, Mauricio et al. 1998) combines gas production profiles and both rate and extent of degradation to evaluate feedstuffs in vitro. The simplicity, low initial cost and high capacity of the system (75 treatments with 4 replicates plus controls can be examined per run) allow it to be widely applied. However, and in common with many other in vitro systems, little information had been generated regarding either its sensitivity or precision. This paper describes how pure substrates were used to identify the minimum amount of fermentable material required to develop distinct degradation profiles. Substitution techniques examined whether changes in degradability at either the mono- or polysaccharide level could be detected and quantified, and replication of these studies provided data regarding precision of the system.