In the design of high-speed rotating machinery, engineers are seeking ways to enhance rotordynamic stability margins while minimizing vibration levels. An attractive option to improve stability margins is the use of annular seals. These seals are versatile because they are effective sealing elements, possess both stiffness and damping characteristics, and are easily incorporated into a pump. Design of an effective seal requires insight into the parameters that can influence its rotordynamic characteristics. Analysis indicates that changes in clearance and taper can significantly affect direct stiffness as well as other seal characteristics. With this in mind, test results are presented for seals with identical round-hole pattern stator surface treatments at two constant radial clearances and one tapered-seal configuration. Testing is performed with a realistic, high inlet fluid preswirl condition that enables an evaluation of a given seal’s stabilizing capacity. All test results presented are for a centered seal condition at multiple shaft speeds and pressure levels. Experimental trends are consistent with theory in that gains in direct stiffness are realized with both a decrease in a seal’s radial clearance and with a convergent-taper. Also, the stabilizing capacity of the seals was significantly influenced by the configurations tested.