The selectivity of the formation of glycerol from saccharides and hexitols by catalytic hydrogenolysis is determined by a number of reaction variables. Evaluation of the results of variations in operating conditions was effected by the use of the parameters: cleavage selectivity, hydrogenation selectivity, and cleavage percentage. There were strong indications that the cleavage selectivity was controlled by the molecular structure of the saccharide used, being considerably higher for a fructofuranose structure than for a pyranose one. The main reactions were cracking, isomerisation, acid formation, hydrogenation, dehydrogenation, and dehydration. Of these, cracking and hydrogenation reactions had the greatest influence on the glycerol yield. The choice of the catalyst is difficult, because of the conflicting demands for maximum hydrogenation selectivity and maximum cleavage percentage.