The isoflavone constituents in soybean seeds were investigated, and 9 kinds of isoflavone glycosides were isolated from the hypocotyls of soybean seeds. Three kinds were proved to be malonylated soybean isoflavones named 6"-0-malonyldaidzin, 6"-O-malonylglycitin and 6"-0-malonylgenistin by UV, MS, IR and NMR.The malonylated isoflavone glycosides as major isoflavone constituents in soybean seed were thermally unstable, and were converted into their corresponding isoflavone glycosides. All of the isoflavone components produced intensely undesirable taste effects such as bitter, astringent and dry mouth feeling. Soybeans are known to contain the five isoflavone glycosides, genistin,^daidzin,1} glycitein 7-O-jS-D-glucoside (glycitin),2) 6"-O-acetylgenistin3) and 6"-(9-acetyldaidzin,4) and