The report gives results of an EPA radon mitigation project involving 14 slab-on-grade houses in Polk County, FL, having indoor radon levels of 320-3810 Bq/cu m (8.7-103 pCi/L), using sub-slab depressurization (SSD) in a variety of applications to evaluate optimal design criteria to be recommended as cost-effective and capable of reducing indoor radon concentrations in houses built over compacted soil fills. For all houses, obvious accessible radon entry points were sealed, and 53-90 L (12-20 gal.) suction pits were dug into the fill material. Two of the houses were mitigated with exterior horizontal suction holes drilled through the stem walls. In four houses, one or more suction pipes were in the garage. The remainder of the interior suction holes were in closets or some other unobtrusive location. Except for the two houses with exterior systems, the other 12 had mitigation fans in the attic. In-line centrifugal fans were used to mitigate each house, although a larger radial blower was installed overnight for experimental purposes in one house, and a vaccumcleaner was used to simulate a larger suction in another house for pressure field measurements only. Post-mitigation worst-case radon concentrations in these houses ranged from 40 to 290 Bq/cu m.