The separation of forages into a soluble fraction like cell contents and into an insoluble matrix like cell walls is an initial step in forage evaluation. Further analysis can be applied to study fermentation characteristics of soluble and insoluble fractions. Unfortunately, most laboratory in vitro techniques quantify incubation insoluble residues and are therefore inappropriate for the examination of food solubles. In vitro gas methods (for example Menke et al., 1979) reflect the production of short chain fatty acids (SCFA) and are therefore free of the disadvantages associated with gravimetric methods. However, it was recently pointed out that an inverse relationship may exist between SCFA production and microbial biomass yield (for review see Blümmel et al., 1997a). These authors suggested combining in vitro gas measurements with the quantification of substrate concomitantly degraded to calculate a partitioning factor (PF). This factor reflects the substrate-dependent variation in SCFA production and microbial biomass yield; the factor (PF) was shown to be valuable in voluntary food intake (DMI) predictions (Blümmel et al., 1997b). In the work presented here, the rate and extent of fermentation of whole roughages and extracted neutral-detergent fibres (NDF) of 54 roughages were examined in an in vitro gas test. The dry-matter degradability of extracted NDF was also quantified and the partitioning factor for NDF (PFNDF) was determined. The relevance of these findings for the analysis of extracted NDF and cell solubles is discussed and their implications for the prediction of DMI of roughages by gas tests are addressed.