Abstract. Batch cultures of rumen microorganisms, using rumen fluids from 4 ruminant species, sheep, goats, cows, and deer, were used to study the ability of polyethylene-glycol (PEG 6000) to inhibit the effect of 2 types of tannins, quebracho (QUE, a condensed tannin) and tannic acid (TA, a hydrolysable tannin) on several in vitro rumen fermentation characteristics. Both QUE and TA were able to impair ruminal fermentation (they reduced gas production, extent of degradation, ammonia-N, and volatile fatty acid concentrations, etc.; P < 0.05), with differences depending on the inoculum donor. The clearest effect of tannins was the reduction of the rates of fermentation, which was observed in all species (P < 0.05). The detrimental effects of tannins were removed by the presence of PEG in most cases, but there were important variations and noticeable exceptions. Thus, for instance, PEG failed to revert the negative effect of TA on the rate of fermentation and the extent of degradation (P < 0.05). The extent of the limited ability of PEG to completely inhibit the negative effects of tannins on in vitro ruminal fermentation seems to depend both on the type of tannin and the species of the rumen inoculum donor.