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 twolevel rules are reversible and that grammars ofnnilevel 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.