The microbial fermentation of the tropical leguminous browses Acacia cornigera (ACA), Albisia lebbekoides (ALB), Enterolobium cyclocarpum (ENT) and Leucaena leucocephala (LEU) was estimated in vitro as the final production of gas. ACA gave the greatest final (asymptotic) gas volume (D; 179 ml/g DM) and fractional rate constant (0.070). Although ENT showed the greatest rate constant (0.87), reaching half its maximum gas production in 7.8 h, it produced less gas than ACA by 9 h incubation, and did not differ from LEU by 24 h ALB gave the lowest gas production. In another trial the negative effect of tannins was estimated over 48 h as ameliorated by the addition of PEG. The effect was variable between species, the greatest being with ALB (Restriction of 0.60-0.68 of the potential gas volume due to the presence of tannins) and increasing slowly, reaching its’ maximum at 12 or 24 h with ACA (0.33) or LEU (0.16). There was no response to the addition of PEG with ENT indicating that other secondary compounds distinct from the condensed tannins were affecting microbial utilisation.