Experimental analysis has been performed, using the computer-controlled Scanning Minipermeameter (SMP) at the New Mexico PRRC, to determine many of the operating characteristics and parameters of these devices. In a minipermeameter, the steady-state pressure drop and flow rate are measured as a gas is injected through a small tip at constant rate into the face of a porous rock. The gas emerges through the surrounding area on the same face. The SMP uses tips molded from silicone rubber that are confined laterally in brass tip bodies that help to maintain constant the outer and inner dimensions of the sealing surface when a tip is in use. Extensive tests with this apparatus on Berea sandstone are reported here. After preliminary repeated experiments to verify that the SMP gives realistic and repeatable results, comprehensive experiments were performed to determine the influence of the force with which tips are held against the rock sample. Additionally, we determined the radius and depth of the ＇＇region of investigation＇＇ - the cylindrical volume beneath the tip, outside of which variations of the rock＇s permeability affect the measurement by less than 1%. These distances are given as dimensionless numbers (fractions of the outer diameter of the tip) and are functions of the ratio of the tip＇s outer to inner radii. It is expected that these considerations would apply to minipermeameters in general, and not only to the SMP. Several strong recommendations are also made concerning the use of minipermeameters, by which more reliable and consistent readings may be obtained. With suitable precautions, minipermeameters can be used to measure permeabilities accurately and inexpensively. These instruments have the additional advantage that very small sample regions can be examined, so rock heterogeneities can be more accurately observed and described.