ABSTRACT:An attempt was made to establish a novel electron micrographic method (aslant sliced thin sectional (ASTS) method) for evaluating pore characteristics, such as mean pore radius, pore radius distribution and porosity on inner layer at a given distance from the outer or inner wall of a porous polymeric hollow fiber membrane and to apply the method for clarifying the effect of preparatory conditions of the membrane on pore characteristics. Regenerated cellulose, cellulose diacetate, poly(vinyl alcohol) and polypropylene hollow fiber membranes for electron microscopic observation were sliced obliquely along the fiber axis at an angle 0 (0<0<2.5") to give a thin section with a thickness less than I µm. The method affords us the information on continuous changes in pore characteristics along the radial direction, where the normal line of the section intercepts the fiber axis at angle 90-0 degree. The electron microscopic image obtained for a portion of an aslant sliced thin section could not be distinguished from those obtained by a thin section cut strictly parallel to the fiber axis (i.e., 0=0). The ASTS method has the following advantages: (I) The location of a sliced section film can be evaluated accurately; (2) only a single operation to slice the sample section is needed; and (3) information on pore characteristics can be obtained continuously along the membrane thickness.