The catalytic oxidations of chiral allylic alcohols 2 by manganese complexes of the cyclic triamine 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn) 1 and hydrogen peroxide as oxygen donor in the presence of co-catalyst are investigated to understand the factors that affect the catalyst selectivity. Chemoselectivity and diastereoselectivity of catalyst 1 are significantly affected by the structure of the allylic alcohol and the nature and amount of co-catalyst. More pronounced is the influence of the amount of added molar equivalents of H(2)O(2) (20-110 mol % with respect to the substrate). Our present results reflect the complex redox chemistry of the Mn catalyst 1/H(2)O(2)/co-catalyst system in the early phase of the alkene oxidation.