Alkaloidal components in the following plants of Celastraceae were examined: Euonymus Sieboldiana Blume (japanese name, Mayumi), Euonymus alata Sieb. forma striata Makino (Komayumi), Euonymus alata Sieb. (Nishikigi), Euonymus oxyphylla Miq. (Tsuribana), Euonymus japonica Thumb. (Masaki). A crystalline alkaloid was isolated from Nishikigi. Two crystalline alkaloids, evonine-I (1) and -II (2) were obtained from Mayumi. Structural studies on evonine-I (1) and -II (2) were carried out. Evonine-I (1) and -II (2) were interconvertible. Evonine-I (1) contains two tertiary methyls, a hydroxyl, a ketone carbonyl, and ether (an acetal), two esters due to evoninic acid, and five O-acetyl groups. Hydrolysis of evonine-I (1) afforded evoninic acid (13). From a series of reactions evonine-I (1) was proved to be consisted of evoninic acid (13) and a highly oxygenated C-15, through two ester linkages. By means of chemical and spectral evidence interrelationship of most of the functional groups in evonine-I (1) was established. Further, the partial structure of evonine-I (1) was deduced.