The pore size distribution of ultrafiltration membranes can be determined using the liquid displacement method developed by Munari and coworkers. However, when this technique is employed, several phenomena have to be taken into consideration in order to avoid erroneous results. In this paper three phenomena are discussed. 1) During the measurement the liquid flow is increased stepwise, resulting in pressure instabilities at the membrane surface. 2) The membrane permeability may be dependent on the pressure and has to be determined at several pressures. 3) Successive measurements of the same module show large differences between the first run and following runs. Traces of glycerol, incomplete pore filling, or long-term swelling are thought to be responsible for erroneous results in the first run.