The Porters Creek Formation, a Tertiary age formation found in the Gulf Coastal Plain region, is known to exhibit shrink/swell behavior. This formation consists of mixed-layer clays composed mostly of montmorillonite. However, to date, the Porters Creek Formation has been ranked as only moderately expansive compared with other expansive geologic materials. During this investigation, surface and subsurface samples of the Porters Creek Formation were subjected to a variety of geotechnical tests to determine their shrink/swell potential. The tests included Atterberg limits, percentage of clay, activity, and volumetric change of samples using a potential volume change meter. The results were used to rank the shrink/swell behavior of the samples in six commonly used classification systems that are based on empirical relationships between the engineering properties of the clay mineral constituents. The same classification systems were then used to classify other geologic formations known to exhibit shrink/swell behavior; then, these results were compared with the results of analyses of the Porters Creek Formation. The samples from the Porters Creek Formation generally ranked in the highest or second highest category of shrink/swell potential, higher than generally expected. This indicates that the Porters Creek Formation has the same high potential for damaging shrink/swell behavior as other formations known to cause damage to roads and other engineered construction, and the current ranking of the Porters Creek Formation by the Federal Highway Administration is too low.