A simple and green method that uses [Ru(Me3tacn)Cl3] (1; Me3tacn = N,N',N''-trimethyl-1,4,7-triazacyclononane) as catalyst, aqueous H2O2 as the terminal oxidant, and Al2O3 and NaCl as additives is effective in the cis-dihydroxylation of alkenes in aqueous tert-butanol. Unfunctionalized alkenes, including cycloalkenes, aliphatic alkenes, and styrenes (14 examples) were selectively oxidized to their corresponding cis-diols in good to excellent yield (70-96%) based on substrate conversions of up to 100%. The preparation of cis-1,2-cycloheptanediol (119 g, 91% yield) and cis-1,2-cyclooctanediol (128 g, 92% yield) from cycloheptene and cyclooctene, respectively, on the 1-mol scale can be achieved by scaling up the reaction without modification. Results from Hammett correlation studies on the competitive oxidation of para-substituted styrenes (rho = -0.97, R = 0.988) and the detection of the cycloadduct [(Me3tacn)ClRuHO2(C8H14)]+ by ESI-MS for the 1-catalyzed oxidation of cyclooctene to cis-1,2-cyclooctanediol are similar to those of the stoichiometric oxidation of alkenes by cis-[(Me3tacn)(CF3CO2)Ru(VI)O2]+ through [3+2] cycloaddition (W.-P. Yip, W.-Y. Yu, N. Zhu, C.-M. Che, J. Am. Chem. Soc. 2005, 127, 14239).