In lieu of an abstract: The future founder of the Social Democratic Party (Sosyal Demokrat Fırkası, SDF), Doctor Hasan Rıza’s (bin Esat) relationship with the socialist movement began during the Second Constitutional Period.1 A permanent member of the Medical Society, Dr. Hasan Rıza resigned from the Committee of Union and Progress (CUP) with a letter he wrote in January 1911.2 He subsequently adopted socialism, and in 1913–1914 contacted the Socialist International to inquire how to become a member of the organization.3 The SDF, however, was not established during this period, but following World War I, which ended in defeat for the Ottoman Empire. The socialist parties and groups founded at the beginning of the Second Constitutional Period could not survive in the authoritarian political atmosphere that emerged after the assassination of Mahmut Şevket Pasha in July 1913 and could only begin a reorganization process once the CUP government, which had been ruling unilaterally since January 1913, fell from power. At the end of 1918, the SDF was among the first leftist parties to emerge during this period.4 In addition to Dr. Hasan Rıza, who was listed as the leader of the party in its founding application, twelve other people are mentioned (fig. 1).