Ruminant production accounts for 81% of greenhouse gas emissions in the livestock sector. Of this quantity, enteric fermentation methane accounts for 90%. Dietary supplementation with tannins is known to mitigate methane production, but also affect feed digestibility. The aim of this study was to investigate the effect of ellagic acid (EA) and gallic acid (GA), alone or in combination, on rumen fermentation in a short-term in vitro experiment using Hohenheim Gas Test. EA and GA were supplemented to a control diet (hay, 200 mg DM). Five different conditions were applied to this study (% of DM): (1) EA 7.5; (2) EA 15; (3) GA 7.5; (4) GA 15; and (5) EA 7.5 + GA 7.5. After an incubation of 24 h at 39 °C, pH, ammonia formation, gas production, short-chain fatty acids (SCFA), in vitro organic matter digestibility (IVOMD) and the microbial count were determined. The treatments did not alter microbial count and pH. Total SCFA production decreased by approximately 10% after all treatments (P<0.001). In the ruminal SCFA profile, the differences were still significant (except for valeric acid) but less evident than total SCFA. Total gas production slightly decreased (-10%) after all treatments, except for GA 7.5. EA 15 and EA+GA treatments decreased methane production per g DM by 20 and 25%, respectively. These two treatments decreased by 10% the production of CO2 per g DM and the production of ammonia on DM basis, respectively by 13 and 20%. All the treatments caused a slight but significant 10% reduction of IVOMD. In conclusion, diet supplementation with EA and GA, alone or in combination, may be a promising dietary strategy to mitigate methane production in ruminants. Further long-term in vitro ruminal fermentation studies are needed to validate these results, before assessing these treatments in vivo.