To what extent has the Argentine party system been polarized along class lines? The political historiography gives mixed and contradictory answers to this question. We explore the social bases of Argentina's political parties using an original database, the most comprehensive database of Argentine elections yet assembled, and new methods of ecological inference that yield more reliable results than previous analyses. We identify two distinct party systems, one in place between 1912 and 1940, the other emerging after 1946. The first party system was not consistently class based, but the second was, with the Radical Party representing the middle classes and the Peronists, workers and the poor. Still, there were important exceptions. Lower-class support for the Peronists, as proxied by literacy rates, declined during Perón's exile, which implies that the party had trouble mobilizing lower-class illiterate voters. Since the return to democracy in 1983, class polarization has again found some expression in the party system.