Flexible-wall permeability (hydraulic conductivity) tests are conducted on compacted specimens consisting of 50 percent Class F fly ash with a 50 percent soil admixture (sand with 0, 6.25, 12.5, 18.75, and 25 percent bentonite). All test specimens are back-pressured before permeation with distilled water. Three specimens from each sand-bentonite-fly ash mixture are tested under high (72 or 73) and low (6.1) constant-gradient conditions as well as under constant-flow conditions for a total of 15 tests. Induced hydraulic gradients from the constant-flow tests range from 3.5 to 41. A decrease in permeability with an increase in bentonite content is evident regardless of the test method. However, the permeability is minimized at a bentonite content of 18.75 percent regardless of the test method. The overall lowest permeability value is 1.6 × 10-9 m/s. An increase in hydraulic gradient of 11.9X resulted in a 3.1X to 6.2X decrease in measured permeability values for all test specimens. A significant portion of the difference in measured permeability values is attributed to differences in applied or induced hydraulic gradients regardless of the test method.