Abstract: This paper constitutes an investigation into the generative capabilities of two-level phonology with respect to unilevel generative phonological rules. Proponents of two-level phonology have claimed, but not demonstrated, that two-level rules and grammars of two-level rules are reversible and that grammars of unilevel rules are not. This paper makes "reversibility" explicit and demonstrates by means of examples from Tunica and Klamath that two-level phonology does have certain desirable cababilities that are not found in grammars of unilevel rules.