The Hagia Sophias of Istanbul, Iznik and Trabzon shared similar conversion histories. All three were built as Byzantine churches, converted into mosques under the rule of the Ottoman Empire, and functioned as museums in the 20th century. Transforming such emotionally charged spaces, either into buildings reserved for the practices of another religion or into public museums open for visitation, requires major physical and conceptual changes, which are closely related to the political, historical and social contexts in which they take place, and are deeply embedded in long-term contestation over these sites. The Hagia Sophia of Istanbul has been the site of prolonged controversies since it was closed as a mosque in 1931 and opened as a museum in 1935. Both Muslims and Christians venerate the museum as sacred, and some groups have been demanding its opening for religious practice. The debates over these museumified sacred sites have increased through recent developments. Unexpectedly, in November 2011, the Hagia Sophia of Iznik was opened as a functioning mosque. In January 2013 the Directorate of Pious Foundations announced plans of converting the Hagia Sophia of Trabzon back into a mosque. This paper focuses on the debates around the museumification and de-museumification of these emotionally charged buildings, analyzing the historical, political, social, religious, and institutional factors in the manifestation of major structural and conceptual changes related to the museumification and de-museumification practices.