The identification and quantification of the relative importance of reproductive isolating barriers is of fundamental importance to understand species maintenance in the face of interspecific gene flow between hybridising species. Yet, such assessments require extensive experimental fertilisations that are particularly difficult when dealing with more than two hybridising and long-generation-time species such as oaks. Here, we quantify the relative contribution of four postmating reproductive isolating barriers consisting of two prezygotic barriers (gametic incompatibility, conspecific pollen precedence) and two postzygotic barriers (germination rate, early survival) from extensively controlled pollinations between four oak species ( Quercus robur , Quercus petraea , Quercus pubescens and Quercus pyrenaica )that have been shown to frequently hybridise in natural populations. We found high variation in the strength of total reproductive isolation between species, ranging from total reproductive isolation to advantage toward hybrid formation. As previously found, Q . robur pollen was unable to fertilise Q . petraea due to a strong reproductive isolating mechanism. On the contrary, Q . pubescens pollen was more efficient at fertilising Q . petraea than conspecific pollen. Overall, prezygotic barriers contribute far more than postzygotic barriers to isolate species reproductively, suggesting a role for reinforcement in the development of prezygotic barriers. Conspecific pollen precedence reduced hybrid formation when pollen competition was allowed; however, presence of conspecific pollen did not totally prevent hybridization. Our results suggest that pollen competition depends on multiple ecological and environmental parameters, including species local abundance, and that it may be of up- permost importance to understand interspecific gene flow among natural multispecies populatins