Stationary-phase evaluation in reversed-phase liquid chromatography (RP-LC) is not a straightforward process. A number of tests to characterize and classify stationary phases have been suggested. The results of these various tests, however, do not always describe the real properties of the stationary phase. This study critically compares several tests for RP-LC stationary phases, including the Engelhardt, Tanaka, and SRM 870 tests, as well as an in-house test, with emphasis on the stationary-phase descriptors of hydrophobicity and silanol activity. The stationary phases were prepared by thermal immobilization of poly(methyloctylsiloxane) onto silica. Hydrophobicity data from the tests were generally good and interchangeable between the several tests. In contrast, the silanol activity results of the various tests differ significantly. As a consequence, stationary phase classification with respect to silanol activity depends considerably on the test method applied. A new classification method for silanol activity is proposed.