The surface films on samples taken from 2 irreversibly blocked reverse osmosis membranes were investigated chem. and microbiol. In both cases biofouling was proven to be the cause of blocking; in 1 case there was also addnl. scaling with Fe(OH)3. Expts. on membrane cleaning showed, that even after 2 days of action of cleaning agent, 20% of the film remained on the membrane surface. This may provide an explanation for the short period of time elapsing between cleaning and performance drop as obsd. in practice: the remanents of the film promote the development of a new biofilm. It must be taken into account that performance parameters allow the detection of biofouling only in a late phase of the process. The age of the biofilm is of great importance for the cleansing capacity of membrane cleaners. It is recommended that prophylactic cleaning be carried out before operation parameters indicate a performance drop.