A method and a model, based on Poiseuille flow through membrane pores, first presented by Christensen et al. [L.H. Christensen, J. Nielsen, J. Villadsen, Chem. Eng. Sci. 46 (1991) 3304-3307] were modified to determine the pore size distribution of a tubular microfiltration membrane. The pore size distribution was calculated from the response signal in the permeate which was generated by a step input signal in the concentration of L-ascorbic acid in the feed. The concentration of L-ascorbic acid was measured with an HPLC-UV detector. The measurements showed that the response signal differs depending on the condition of the membrane: measurements were done for a clean membrane, the same membrane with a fouling layer consisting of milled paper, and the same membrane after the fouling layer had been removed. The presented model can describe the pore size distribution of the clean membrane and the membrane after the fouling layer has been removed. With respect to the model, the influence of different parameters on the response signal is discussed.