Abstract: Clay mineralogy of the sediments deposited since the Pleistocene in the Mariana Trough and the West Philippine Basin was investigated by means of X-ray diffractometry and chemical analysis, based on three sediment cores (61KL, 57KL, 85KL) collected during the "SONNE"Cruise 57 in 1988. Predominant, well-cyrstallized smectite, with certain amount of Fe Mg chlorite, small amount of illite and no kaolinite, were identified in the sediments of the Mariana Trough (61KL, 57KL). Smectite and Fe Mg chlorite are authigenic in origin and were mainly formed by submarine alteration of basic volcanic material in the Trough, whereas illite is detriral in origin and was mainly derived from the continents west of the Philippine Sea. In the sediments of the West Philippine Basin (85KL), however, illite is predominant, followed by certain amount of poorly-crystallized smectite, small amount of normal chlorite and kaolinite. These minerals are chiefly detrital in origin and were mostly derived from the continents west and northwest of the Philippine Sea, except for smectite which may be mostly derived from the alteration of basic volcanic material on the Palau-Kyushu Ridge east of the basin, but at least small part of the smectite may probably be derived from the nonvolcanic rocks on the land. The main factors affecting the associations and distributions of clay minerals in the Mariana Trough and the West Philippine Basin were discussed. The volcanic activity is the most important factor controlling the clay mineral associations and distributions of the two basins. Material sources also play an important roll. The variations in chemical characteristics of clay minerals found in the Mariana Trough may reflect the influence of submarine hydrothermal activities possibly present in this basin. The changes in paleoclimate is an important factor controlling the clay mineral distributions, but in the Mariana Trough such an influence is seriously masked by the frequent volcanic activities.