Two studies were conducted to screen leaves of four browse species (Dichrostachys cinerea, Flagea villosa, Harrisonia abyssinica and Piliostigma thorningii): 1. to quantify levels of total extractable phenolics (TEP), extractable tannins (TET), total condensed tannins (CT), extractable and bound CT fractions and proanthocyanidins (PAs) using chemical assays; 2. to investigate effect of tannin anti-nutritive activity on in vitro gas production assessed by polyethylene glycol (PEG) tannin bioassay. Crude protein (CP) varied (P<0.05) between fodder species from 109 (P. thorningii) to 160 g/kg DM (D. cinerea). The fodders had detectable TEP that varied from 112 (P. thorningii) to 234 mg/g DM (F. villosa). TET varied (P<0.05) between species from 95 (P. thorningii) to 220 mg/g DM (F. villosa). The content of CT varied (P<0.05) from 53.2 (F. villosa) to 98.3 mg/g DM in P. thorningii. High proportion of CT was bound to protein (40-51.4%) compared to soluble (19.5-33.1 %) and fibre-bound (22.7-27.3%) CT fractions. Characterization of PAs in leaves revealed presence of flavan-3-ol and flavan-3,4-diols flavonoids. The increase in GP due to PEG supplementation varied (P<0.05) from 44.4 ml/g OM (14.5%) to 132.3 ml/g OM (69.4%) at 16 h, and from 34.9 ml/g OM (10.3%) to 132.2 ml/g OM (57.2%) at 24 h, in H. abyssinica and D. cinerea, respectively. Variable responses on gas production between species’ leaves due to PEG supplementation demonstrate adverse effects of tannins on depressed feed digestibility. Improved gas production and digestibility were due to the ability of PEG to bind and complex tannins, and recover feed nutrients bound by tannins. Therefore, utilization of these browse species’ leaves as protein supplements to ruminants could be optimized through reduction of levels of phenolic and tannin anti-nutritional factors (ANFs).