In order to find the pressure dependence of water permeability for some flat-sheet ultrafiltration membranes, wet and dry membranes have been precompressed mechanically without allowing any water flux to pass through at pressures between 0 and 0.6 MPa. Thereafter, water at a fixed transmembrane pressure has been filtered and the flux as a function of compaction has been measured. For comparison, membranes have also been compressed hydrostatically, that is with water passing constantly through the membranes at different pressures. At corresponding pressures, the mechanical treatment lowered the flux much more than did the hydrostatic treatment. Cellulose acetate membranes displayed more elastic than viscous behaviour, polysulfone membranes the contrary. The flux was never restored after compaction, and the flux reduction increased with increasing precompression pressure. The glass transition temperature did not seem to have any influence on the compressibility in the present temperature range, but the porosity and the membrane structure were important. The membranes’ thickness change could be correlated to the applied pressure as Delta l similar to Delta P-0.8, a relation typical for porous materials.