Chemical compositions of segregation veins in basalt flows are compared with those of their host rocks for tracing fractionation trends of magmas after extrusion. In some tholeiite of Hawaii and Japan and in Warner high-alumina basalt of California, total iron increases markedly during fractionation while SiO2 is nearly constant. In tholeiite near Catania, Sicily, both total iron and SiO2 increase during fractionation. In high-alumina basalt of Huzi Volcano, Japan, and in some basaltic andesites of the hypersthenic rock series or calc-alkali rock series of California and Japan, total iron is nearly constant or decreases slightly while SiO2 increases. The trend with marked increase of iron is seen in flows with lower Fe2O3/FeO ratio, whereas that with constant iron is seen in flows with higher Fe2O3/FeO. Thus the different trends may arise largely from difference in oxygen partial pressure of the magmas during fractionation. The higher oxygen pressure may be caused by higher initial water content of the magmas. It is also noticed that residual liquids having the chemistry and mineralogy of the hypersthenic rock series originate only from magmas which separate orthopyroxene during extrusion. This fact is also explained as due to the high water content of the magma of this series by which the crystallization temperature is lowered.