This paper deals with the characterization of four hydrophobic microfiltration membranes from water permeation measurements. A method proposed by McGuire et al. is used. The method uses a mathematical technique of smoothing spline fit in order to obtain a smooth pore distribution function without the need of a priori assumptions about the nature or shape of the distribution. Good agreement between the discrete distribution determined by a classical method and the continuous distribution determined from the McGuire method was obtained.