This article analyzes the reform patterns of the ISI model in the earliest modernized countries in Latin America: Argentina, Chile, and Uruguay. I argue that the intertwining of countries’ socio-political matrices and their patterns of reform yield three reformist routes: a populist route, an authoritarian route, and an institutionalized-democratic path. The three selected countries experienced different patterns of reform which could be explained by the effects of
socio-structural factors (constituencies, incorporation legacies) and of institutional factors (patterns of political competition, types of party organization), both of which give way to specific patterns of political representation. I argue that specific national configurations areable to neutralize the inertial effects of structural conditions that would initially favor the reproduction of the ISI model or its gradual reform: societies highly modernized, universal welfare states, and highly mobilized popular sectors. The transformation of ISI constituencies, leftist or populist party organizational change and the mutation of political representation patterns explain the different patterns of reform observed in each country. My causal argument is based on conjunctural and diachronic path-dependent narratives of each case.