Through the sixteenth and seventeenth centuries, Russian captives played a significant role in Ottoman social life, in fact becoming 0ttoman through processes of conversion which scholars have begun to examine. But over the course of the eighteenth century, Russo-Ottoman treaties increasingly required the return of Russian subjects to Russia. This paper traces the implementation of these treaty terms, focusing on how "Russianness" was defined and constrained, and on the strikingly modern lines of legal sovereignty which resulted. I begin in the 1740s, arguing that the abolition of ransom led the two states to emphasize captives' political identity rather than their economic value. Moving on through the remainder of the century, I trace the shifting boundaries of which slaves were, and were not, eligible for Rssian state aid. This was not, however, simply a matter of a victorious Russia dictating terms to the prostrate Ottomans; in fact, I show, Russian pretensions on non-Russian subjects, especially Georgian, were turned back by Ottoman resistance. In the end, I argue, the lines of sovereignty were drawn in dialogue between the two empires’ interests and ideologies. The anti-climactic result of this process was found in the early nineteenth-century Serbian and Greek revolts, in which Russia protested Ottoman conduct, but made no attempt to secure the release of those Ottoman subjects enslaved as a result of these conflicts--suggesting the enduring power of eighteenth-century definitions of sovereignty. The paper, then, provides a prehistory of nineteenth century foreign diplomats' efforts to abolish the Ottoman slave trade, illustrating the very different realities of the eighteenth century while also demonstrating that strikingly modern concempts of international law could arose in the Ottoman-Russian milieu.