Much scholarly attention in the last decade has been devoted to post-conflict constitutional processes and their impact on the overall transition. However, such an approach neglects the vital proto-constitutional phase which inevitably precedes the election or selection of a constituent assembly. This short period is often governed by unelected transitional administrations which fundamentally shape the nature of the constitutional and political processes which follow. The High Commission for the Realisation of the Objectives of the Revolution, Political Reform and Democratic Transition in Tunisia, and the National Transitional Council in Libya are two such transitional administrations. After explaining the structure and make-up of these bodies, I will argue that variation in the transitional trajectory taken in Tunisia and Libya can partly be attributed to the decisions and decision-making processes chosen by these bodies. These factors determine the opportunity and threat structures perceived by different domestic actors and constituencies. I suggest that decisions, particularly concerning the choice of electoral system and candidacy criteria for elections, significantly condition the actors who secure decision-making roles and constituencies which are represented in the political sphere. This impact both the policy direction taken and the character of the State as a whole. Furthermore, in establishing the putative “rules of the game”, particularly who is de facto excluded from participation, these transitional administrations significantly impact upon the acceptance and legitimacy of the transition, and may determine the State’s success or failure to emerge from conflict and unrest. For example, the decision to adopt a system of proportional representation in Tunisia for the 2011 National Constituent Assembly (NCA) elections ensured that no Party was able to govern alone and dominate the constitution-drafting process. This attenuated concerns about Ennahda dominance during the transitional phase. Conversely, the decision to opt for a largely individual-based electoral system, over a list-based system, in the 2012 General National Congress (GNC) election in Libya created intense fragmentation in the GNC. This fragmentation paralysed the GNC and led to its ineffectiveness, with concomitant effects on the GNC’s legitimacy and acceptance by the population. This ultimately led to the need for two further national elections in the ensuing two years (2013 and 2014) with disastrous results for post-revolutionary Libya. The study’s methodology is qualitative, based on extensive interviews with the principal decision-makers inside these two bodies. Additional primary sources include contemporaneous newspaper articles and interviews, archived documents and public materials, memoires and biographies of key figures.