Mineraiogical studies of; the ;Duchuantou ".illite" in Zhejiang province indi- cate that the mineral formerly called "illite" is actually sericite. Its XRD, IR, DTA, SEM and TEM data and, es-pecially, chemical composition exhibit chara- cteristics .of the typical sexioite mineral. The Duchuantou sericite is characterized by Mg0<0.25%，K20>9%, Si/Al< 1.2, octahedral Mg less than 0.03, and octahedral electric charges -0.052- + 0.048/ O10(OH)2, whereas illite is noted for Mg0>2.0%，K20 some 8%，Si/Al> 1.5, octahedral Mg 0.2-0.4, and octahedral electric charges -0.17一-0.40/O10(OH)2. The Duchuantou sericite.has high crystallinity, flaky form, grain size smaller than 5μm, sharp and symmetric 9.95A, (002) peak, and 2 M polytype, while illite generally has low crystallinity, 1 M polytype and gentle (001) peal. In addition, the Duchuantou sericite displays characteristic 3624, 3450, 930, 825 and 799 cm-1 infrared absorptions, 45-111.2℃ weak endqthermic valley and 600- 700℃strong endothermic valley; in contrast, illite does not show apparent 825, 799 cm-1 infrared absorption, but exhibits 100℃ strong endothermal valley and 5000℃ and 9000℃ endothermal valley.