In this experimental Study, the boulder-flow interaction associated with the self-aeration process was investigated in a 0.50 m wide flume. Experiments were performed with three different boulder arrangements and four different relative boulder submergences using Reynolds numbers from 5 x 10(3) to 5 x 10(4). A boulder-flow interaction can trigger air-water mass transfer by generating turbulence, a hydraulic jump and plunging jet flow. When the boulders are submerged by the water, the maximum local aeration efficiency tends to decrease rapidly as does the flow resistance. Furthermore, the maximum local aeration efficiency is positively related to the blockage ratio of the boulder(s). The blockage ratio appears to represent the head loss, suggesting a possible interrelation between the aeration efficiency and the energy dissipation.