Perforated and flexible aeration diffusers are widely used in biological wastewater treatment plants. Comparing to other aeration techniques, they have certain benefits in price, durability and replaceability. Bubble size from a diffuser should be small enough to achieve large interfacial area between air and water, and therefore sufficient oxygen mass transfer. Air compressing is a significant part of the operational costs of a wastewater treatment plant. Thus, by optimising bubble size and flow conditions savings can be achieved. These optima can be reached by the development of the membrane construction and by the means of state-of-the-art multiphase flow modelling. For both purposes, local bubble size distribution data is valuable. In the literature, bubble size data measured inside the dense bubble dispersions are scarce. In this work, local bubble size distributions were measured for five industrial membrane diffusers. The variables were hole specific gas flow rate, membrane bole density and hole diameter. Two apparatuses were used: small rectangular vessel of height 0.6 m and rectangular bubble column of height 3.0 m. Axial and radial variation of bubble size distributions was investigated by a capillary suction probe (CSP) and by digital photography. Inside the dense bubble dispersion, where most of the measurement techniques are not applicable, CSP was found to be a successful technique. Bubble size variation was notable, both axially and radially. Increasing the gas flow rate increased the bubble size. Same effect was found when increasing the hole density. The effect of hole size variation was not observed that clearly with these membranes. Minimum in the bubble size was detected 19 cm above the flexible membrane. This indicates that the tailoring of the bubble size should be made according to situation there, not at the surface of the membrane. (c) 2005 Elsevier B.V. All rights reserved.