Scholars of Iranian constitutional history have long recognized the influence of the Belgian and Bulgarian constitutions on the Iranian 1907 constitution. The exact character and extent of these and other constitutional influences have remained unclear, however. This article provides an analytical comparison of the 1907 Supplementary Fundamental Laws with the 1831 Belgian, 1876 Ottoman and 1879 Bulgarian constitutions that served as models and sources of inspiration. We also provide an easily navigable annotated version of relevant constitutional provisions in the footnotes for scholars interested in tracing models for particular provisions and have provided a complete version of the 1907 Supplementary Fundamental Laws and its sources on our website. In doing so, this article and the accompanying materials hope to clarify where these influences begin and end, where they have been modified or ignored, and where Iran’s constitutionalists innovated by introducing more stringent separation of powers or new institutions. It is thereby demonstrated that Iran’s constitutionalists critically engaged with previous constitutional traditions, rather than merely copying provisions from earlier models. Thus, Iran’s 1907 Supplementary Fundamental Laws should be regarded as an organic engagement with and global extension of the European liberal tradition, rather than as a merely peripheral or derivative development.