The Russian and Ukrainian constitutions—like those in many other post-Soviet states—have concentrated political power in exclusive “super” presidencies. However, the concentration of power has persisted in only one of the two cases. Russian presidential authority was resilient in the face of attempts to increase legislative strength in the 1990s, even when severe economic and political crises undermined the presidency of Boris Yeltsin. In contrast, Ukrainian presidential power fluctuated over time, with “Orange Revolution” constitutional reforms shifting power to the parliament in 2004 and their annulment returning power to the president in 2010. What explains the different trajectories of Russia’s and Ukraine’s presidential systems? Using process-tracing to parse out the actions of elites during the 1990s and 2000s in combination with analyses of the electoral foundations of elite competition in the two cases, this dissertation develops an argument about the origins of super-presidential systems and the prospects for constitutional change in such systems. Concentrated executive power in Russia and Ukraine: (1) depended on elites’ preferences for more or less concentrated political authority; (2) these preferences depended on how elites perceived their political prospects for capturing and holding presidential power; (3) elites’ perceptions of their prospects for gaining and holding presidential power were conditioned by the relative balance of power between major political forces; and (4) this balance of power was very vulnerable to pressure from social forces. It was this final factor that distinguished the Ukrainian and Russian cases. Ukraine had more balanced political competition because of its coherent ethno-linguistic cleavage, and consequently more uncertainty about rival elites’ political fortunes, which produced challenges to super-presidentialism. Russia’s experience with regional politics, by contrast, has not produced a similarly stable balance of power between rival forces, because the country’s minority groups were too diverse and dispersed to form a unified constituency that could challenge the political dominance of the center. The structural underpinnings of elite competition help to explain why the preferences of self-interested politicians to concentrate or disperse political power changed over time in ways that promoted unstable super-presidentialism in Ukraine compared to much more durable super-presidentialism in Russia. / text