The results of X-ray diffraction analysis of clay minerals from the Hydrate Ridge portion of the Cascadia subduction zone are consistent with previous studies of near-surface sediments from the continental margin of Oregon and southern Washington. On average, early Pleistocene to Holocene trench slope and slope basin deposits contain 29% smectite, 31% illite, and 40% chlorite (+ kaolinite). Late Pliocene to early Pleistocene strata from the underlying accretionary prism contain moderately greater amounts of smectite with average values of 38% smectite, 27% illite, and 35% chlorite (+ kaolinite). There is no evidence for in situ diagenetic alteration of the clay minerals, but reaction of smectite at greater depths probably contributes to the documented freshening of pore waters.