Magnesium oxide (MgO) contained in steelmaking slags in its liberated form, referred to as free MgO, expands when hydrated. The expansion of free MgO sometimes causes the deterioration of steelmaking slags used as construction materials for roads. Therefore, a method that determines the free MgO content in steelmaking slags is required. Solvent extraction is a promising preparation method for determining free MgO in steelmaking slags. Herein, organic solvents were explored for their extraction capabilities, and MgO was successfully extracted with 2,2,2-trichloroethanol (TCE) at 140°C over 3 h. TCE could not be used to extract any other magnesium compounds in blast furnace slags or merwinite, which contain no free MgO, indicating that free MgO was selectively extracted with TCE. The MgO content determined by TCE extraction and atomic absorption spectrometry was found to be reasonably consistent with that determined by other methods, which implies that this method is suitable for measuring the MgO content in steelmaking slags. The mechanism for the extraction of MgO by TCE was also investigated, and it was found that TCE extraction occurred via an acid-base reaction and an oligomerization crosslinking reaction.