Electoral systems are uniquely distributive political institutions that shape political outcomes, yet are themselves endogenously shaped outcomes of political choices. In Poland, party system development has involved not only parties adapting to electoral institutions in each election, but also parties modifying these institutions prior to every election. We model electoral system change as driven by partisan self-interest in maximizing seat share and test it in five episodes of electoral system change in Poland from 1989 to 2001, comparing parties’ support for electoral law alternatives to their expectations of seat shares from those alternatives. Data consists of opinion polls, roll-call votes, Sejm records, constitutional committee transcripts, and interviews with political actors who designed and chose the Polish electoral institutions. The findings clearly show that party support for each electoral law was closely linked to the perceived effect on that party's seat share, with this linkage growing more consistent over time.