A novel protocol for the practical and green synthesis of vicinal cis-diols from 10.0 mmol olefins by using 5.0 mmol KMnO₄ as oxidant and 30.0 mmol H₂O₂ as co-oxidant is reported. The presented procedure is easy to carry out and enables the direct transformation of linear and cyclic alkenes to the corresponding vicinal cis-diols. The synthesis of vicinal cis-diols by dihydroxylation of olefins with a KMnO₄/H₂O₂ system was catalyzed by in situ generated Mn_xO_y nanoaggregates. The use of H₂O₂ as a co-oxidant is the key for the protocol to synthesize vicinal cis-diols in high yields, because it assists the oxidation of Mn_xO_y nanoaggregates, which have an active role in the oxidation reaction medium.