Purpose: In present work, the mineral phases in chromium-containing slags were identified, and the formation mechanisms of Cr(VI) in Cr2O3-containing mineral phases were discussed. Design/methodology/approach: In steelmaking slag containing chromium, chromium is mainly present as spinel phases such as MgCr2O4 and (Mg,Fe)Cr2O4. When these Cr(III) oxides are oxidized to Cr(VI) oxide, the Cr(VI) is easily eluted from the slag due to its high solubility in water. Findings: The Cr (VI) adversely affects the human health and the environment. In this study, the influence of cooling rate, quenching temperature and oxidation/reduction condition during cooling on the Cr(VI) amount eluted from synthesized MgCr2O4, (Mg,Fe)Cr2O4 and CaCr2O4 were investigated. Research limitations/implications: The formation of Cr(VI) oxide in MgCr2O4 and CaCr2O4 compounds during heating under air was considered to be indispensable. The amounts of Cr(III) and Cr(VI) dissolved from MgCr2O4 were smaller than those from CaCr2O4. Since the formation of CrO3 in MgCr2O4 started at around 1400 K during cooling, slag should be cooled rapidly from the high temperature above 1400 K, or cooled in inert atmosphere in order to minimize Cr(VI) formation. FeO in (Mg,Fe)Cr2O4 solid solution suppressed Cr(VI), Cr(III) and Mg elution. Originality/value: The development of prevention method of Cr(VI) formation in the slags containing chromium is urgent in order to utilize the slags to land-fill and civil engineering works.