Based on mercury porometry and microscopy data, the most suitable types of powdery polytetrafluoroethylene for preparing biporous hydrophobic matrices for chromatomembrane mass-exchange processes were chosen. It was found that there is no sharp boundary between the sizes of the pores through which the polar and nonpolar phases flow during a chromatomembrane process and that the pressure in the polar phase influences the volume fractions of these two phases in the biporous matrix.