This study investigated the role of condensed tannins (CT) in sainfoin (Onobrychis viciifolia) on the rumen fermentation of mixtures of cocksfoot (Dactylis glomerata L.) and sainfoin. Freeze-dried samples of cocksfoot and sainfoin in different proportions (100:0, 75:25, 50:50, 25:75 and 0:100) were fermented in culture bottles containing buffered rumen fluid from sheep in presence or in absence of polyethylene glycol (PEG), a compound that inactivates CT. In vitro true dry matter digestibility (IVTDMD), total gas production (GP) and ammonia concentration in the incubation medium were determined at 3.5 and 24 h of incubation. In the absence of PEG, ammonia production decreased dramatically at 3.5 and 24 h of incubation (P < 0.001) as the proportion of sainfoin increased in the mixture, with a quadratic effect at 3.5 h (P < 0.01). At 24 h of incubation, GP was lower and IVTDMD was slightly greater for mixtures incubated in absence of PEG than in presence of PEG (P < 0.001 and P < 0.01, respectively). It was concluded that the CT in sainfoin are responsible for the synergy between cocksfoot and sainfoin, making it possible to maximize the degradation of plant substrates while minimizing protein degradation and gas losses.