Free and complex forms of accumulated borate in seaweeds were identified by 1 1 B NMR. The NMR spectra of green and red algae showed boric acid peaks mainly. In contrast, brown algae gave two distinctive peaks predominantly containing borate diesters. To identify the moiety coupled to borate, borate complexes with compounds containing vicinal hydroxy groups, such as mannitol, laminarin and alginic acid, were prepared and examined. The results strongly indicated that mannitol formed the main diester complexes. The signal of the complexes was decreased rapidly after storage. Fresh brown algae, therefore, contain large amounts of B-carbohydrate complexes as do higher plants.