This article is devoted to a comprehensive discussion of Jews' conversion to Protestantism in the Ottoman Empire in the cities of Istanbul and Izmir, during the decade before the Hatt-i-Șerif of Gülhane, the first Tanzimat decree in 1839, until the Hatti hümayun decree of 18 February 1856. It also considers the attitudes of the Ottoman authorities and the Jewish communities toward this phenomenon, as well as the extent to which three missionary societies, the London Society for Promoting Christianity among the Jews, the American Board of the Commissioners for Foreign Missions (ABCFM), and the Free church of Scotland mission, succeeded in prevailing upon Jews to give up their faith. The missionaries worked with the millenarian anticipations and looked forward to intensive activity which would result in the conversion of the Jewish masses. To achieve their objectives they relied on evangelical, educational and philanthropic activities, and on medical missions. The article discusses the motives and the social and economic status of converts to Christianity and those given Christian religious instruction, and describes the communities' steps taken against Jewish conversion to Christianity in the 1820-50s. The article concludes that all three missionary societies that operated in the Ottoman Empire acknowledged the fact of their failure to achieve the goal of converting the Jewish masses, and took solace in those dozens who were converted through their efforts.