Optically active myo-inositol 1,4,5-tris(dihydrogen phosphate) 1, which has now been recognized as a second messenger in a new intracellular signal transduction system, has been prepared starting from myo-inositol. The key step, phosphorylation of an adequately protected polyhydroxy derivative, was accomplished by three methods, among which a phosphoramidite method using a new phosphitylating agent, o-xylylene N,N-diethylphosphoramidite, gave the triphosphoric ester in quantitative yield. Optical resolution was effectively realized by derivatization into diastereoisomeric l-menthoxyacetic esters. NMR spectra and optical rotation are shown to depend on the pH of an aqueous solution of compound 1.