Two types of advanced seals for liquid oxygen (LOX) turbopumps were investigated. One was a spiral-groove face seal whose function is to seal high-pressure LOX at the impeller end of the turbopump. The other was a floating-ring, Rayleigh-step, helium buffered seal used to prevent LOX ingress to the turbine side of the unit. For each seal type, two sizes were investigated (50 and 20 mm). A turbine-driven test rig was designed and manufactured, and a test program was completed on the 50 mm floating-ring, Rayleigh-step, helium buffered seal. Significant results were: vaporization in the flow path could cause failure by overheating; therefore, the spiral-groove pumping portion of the seal that provides the fluid film must circulate fluid without disruption if vaporization occurs in the sealing dam. This is successfully accomplished by a pressure-balanced spiral-groove concept that is described. The spiral-groove configuration is affected by turbulence in the fluid film and pressure drops due to fluid inertia at sudden contractions. The net results of these effects are deep grooves, large operating films, and high power loss when compared against seals operating with laminar films. Turbulence and inertia are induced by the high-density and low-viscosity characteristics of LOX. The program clearly pointed out the need to consider system environmental factors such as thermal and centrifugal distortions and rotor vibrations in the seal design.