Knowledge of the pore size distribution of a microfiltration or ultrafiltration membrane is necessary in order to accurately predict membrane performance in any given application. The liquid permeation technique presented in this paper has been used to obtain data for determining pore size distribution for nearly sixty years. However, the mathematical technique presented in this paper to interpret the experimental data has not been used since its conception in the early nineteen-thirties. This paper examines the merits of this mathematical technique which, when coupled with modern computing capabilities and precise data acquisition, can be used to effectively determine accurate, continuous pore size distributions of hydrophobic membranes.