The objective of this study was to evaluate the change in chemical composition, in sacco neutral detergent fibre disappearance and soluble indigestible components of lucerne and almond hulls subjected to three moisture levels (0, 0.25 and 1 g water g −1 feed) and four heating times (0, 1, 20 and 40 h) at 100 °C. Measurements in lucerne and almond hulls included neutral detergent fibre (NDF), acid detergent fiber (ADF), lignin (two methods), in sacco NDF remaining at 48 h fermentation (INSNDF), nitrogen, acid detergent insoluble nitrogen (ADIN) and neutral non-starch polysaccharides (NSP). Optical densities (OD wavelengths: 280, 350 and 420 nm) were measured in extracts of lucerne and almond hulls as an indicator of soluble indigestible components. Low coefficients of variation were found for nitrogen, ash, glucose, total NSP and OD 350 nm measurements for lucerne across all experimental treatments. Low coefficients of variation were also found for nitrogen, cellulose, galactose and total NSP for almond hulls across all experimental treatments. High coefficients of variation were found for ADF, lignin and ADIN for lucerne and almond hulls across all experimental treatments. Optical density measurements, in sacco NDF disappearance and chemical composition (other than NSP) were altered with time of heating for lucerne and almond hulls. The OD readings at 280 nm increased with lucerne and decreased with almond hulls as heating time increased from 0 to 40 h. Chemical changes for lucerne appeared to occur at early heating times when compared with almond hulls yet the extent of chemical changes was greater at longer heating times for almond hulls. The in sacco NDF remaining after 48 h fermentation (INSNDF) tended to reach a plateau as heating time increased for lucerne, whereas INSNDF values continued to increase for almond hulls. Conversely, ADIN reached a plateau with increased heating times for almond hulls but not for lucerne. The highest INSNDF values across heating times were found at the moisture level of 0.25 g water g −1 feed for lucerne and almond hulls. Moisture level had the greatest effect on neutral non-starch polysaccharides which tended to decrease with increasing moisture levels for lucerne and almond hulls. In sacco NDF 48 h fermentation values were positively correlated with time of heating, NDF, ADF, lignin values, ADIN and OD 280 nm for lucerne. Similar correlations were found for almond hulls except INSNDF was negatively correlated with OD 280 nm readings. The INSNDF changes with lucerne and almond hulls were difficult to interpret because of varying OD readings. The OD measurements were high for lucerne suggesting that some compounds are soluble and indigestible. However, the OD measurements were relatively low for almond hulls.