Numerous works in the last decade have analyzed the question of how to compare opportunity sets as a way to measure and evaluate individual freedom of choice. This paper defends that, in many contexts, external procedural aspects that are associated to an opportunity set should be taken into account when making judgements about the freedom of choice an agent enjoys. We propose criteria for comparing procedure-based opportunity sets that are consistent with both the procedural aspect of freedom and most of the standard theories of ranking opportunity sets.The work of R. Arlegi was supported by the CICYT under Research Projects SEC2000-0838 and SEC2003-08105. The work of D. Dimitrov was supported by a Marie Curie Research Fellowship of the Eu...