The transformation of Marxist rebel groups into political parties represents a complex and interesting phenomenon, still, the determinants of their success or failure in this transition remain poorly understood. The aim of the study is to investigate why some Marxist rebel groups successfully transition from rebel groups into a political party while some do not. Through a comparative study of the rebel groups Revolutionary Armed Forces Colombia (FARC) and Ejército de Liberación Nacional (ELN) in Colombia, this paper will analyze the relationship between internal cohesion and its impact on the rebel-to party transition. The hypothesis that is being tested is that rebel groups with low internal cohesion (vertical or horizontal) among members will have a harder time transitioning into a political party, while rebel groups with high internal cohesion will easier transition into a political party. The findings of this study provide strong indications that internal cohesion influences the successful transition from rebel group to a political party. However, the study also provides indications that other factors need to be considered.