The objective of this work was to study the effects of both Klason lignin (KL) and acid detergent lignin (ADL) on in vitro neutral detergent fibre (NDF) digestion (IVNDFd) in an effort to assess if acid labile phenolic compounds affect the rate of degradation (kd). Eighty five forages (lucerne, maize silages and grasses) were analyzed for NDF, ADL, KL and IVNDFd (6, 12, 24, 30, 36, 48, and 96 h fermentations were used for kd estimations). Correlations were estimated among lignin types (KL vs. ADL), lignin and extent of IVNDFd, and lignin type and NDF kd and tested for significance. Within and among all forage types, the correlation between ADL and KL was in general positive when on NDF basis and high and positive when on DM basis (0.77 to 0.90). Within and among all forages, only ADL was consistently negatively correlated with IVNDFd at all time points (-0.54 to -0.94). Correlations among forages for NDF kd and lignin type were not consistent. Among all forages, KL was negatively correlated with IVNDFd and NDF kd. The correlation between IVNDFd and ADL increased as fermentation length increased among all forages. However, the correlation of KL and IVNDFd was greatest up to 48 h of fermentation suggesting that the soluble phenolics affected both the rate and extent of IVNDFd. Unlike ADL, KL disappeared during IVNDFd, and in most forages there was a high negative correlation associated with the difference between KL and ADL (ΔL) and IVNDFd, except for brown midrib hybrids maize and early cut grasses that had a different behaviour. Among forages, a one unit increase in ΔL corresponded to an average 18% decrease in the kd demonstrating that the greater the difference between KL and ADL the lower the rate of digestion.