Local mean interfacial area in gas-liquid bubbly flow is experimentally investigated; data is obtained from the experiments carried out with a double optical sensor intrusive probe and analysed by statistical methods. Local variables are measured in the riser of an external loop airlift reactor in air/water medium. The results are presented in the form of the radial profiles of void fraction, bubble diameter and local average interfacial area at superficial gas velocities ranging from 0.003 to 0.11 m/s. The axial variations of the same parameters are also investigated. The values of local interfacial area obtained from the classical equation ((a) over bar (t)(i) = 6 epsilony/(d) over bar (32)) and from bubble velocity statistical treatment methods agree reasonably well. This shows that the double optical probe is a reliable method for local measurements of interfacial area. Eventually, the maximum angle alpha (0) between the probe vertical direction and bubble velocity vector at the piercing point is evaluated and the influence of gas superficial velocity on alpha (0) is demonstrated.