A study of the effect of fouling on the standard oxygen transfer efficiency (SOTE) of ceramic diffusers of four different pore sizes was conducted at the Monroe, Wis. wastewater treatment plant over a 2-year period. Four pilot test headers were installed in the plant aeration tanks each containing four diffusers of one pore size. Only minor changes in SOTE, bubble release vacuum (BRV), and dynamic wet pressure (DWP) were observed indicating that the fouling phenomenon at Monroe was not progressive. The adverse effects of fouling with respect to back pressure and SOTE were found to be less than might have been predicted from the literature. The cleaning procedure used in the study, involving a combination of high pressure water spraying with liquid acid treatment, followed by additional spraying, resulted in nearly complete restoration of the diffusers’ original characteristics.