Abstract: The invention relates to the production of mannitol by bioconversion with the aid of immobilized micro-organisms as well as to the use of the mannitol and by-products formed in the conversion process. In said process a solution containing a substrate convertible into mannitol is fed into contact with solid carrier particles containing viable mannitol-forming microorganism cells immobilized in and/or on said particles. The solution is reconditioned between successive conversion steps in order to provide conversion of a major portion of said substrate into mannitol.