By treatment with Grignard reagents esters of the diboronic acids (HO)₂B·R′·B(OH)₂ are converted into the diborinic acids RB(OH)·R′·B(OH)R, which are isolated in the form of their bis-2-aminoethyl esters. Acidification of the latter enables some of the free diborinic acids to be obtained. The hydrolysis, oxidation, and dehydration of the diborinic acids are described.