Biofouling is a frequently occurring fouling mechanism in membrane applications. Controlling this phenomenon is a challenge due to the difficulty of cleaning biofouling in spiral wound membrane elements. It is assumed that cleaning can be more efficient when biofouling is in an early stage of colonisation. Therefore a sensitive method has to be available for an early identification of biofouling. The present method, the measurement of the normalised pressure drop (NPD) is not specific for biofouling and is not very sensitive. In this research the feasibility of the specific oxygen consumption rate for detection of the activity of biofilms was investigated in membrane systems. The method has the advantages to be specific for active biomass, applicable in situ, non-destructive and more sensitive than NPD. Three experiments demonstrated that the measurement of the rate of oxygen consumption is potentially a simple, reliable method for the measurement of the active biomass in membrane systems. During one of the experiments the method illustrated the effect of cleaning and the regrowth of bacteria afterwards. The method will be further evaluated and standardized. The relation between the specific oxygen consumption rate and the condition of the biofilm, measured by autopsy of membrane elements, will be further explored. The ongoing research will result in an apparatus and procedure, to be used for biofouling identification during the operation of full-scale and pilot plants.