AbstractThe paper describes a language consisting of two layers, terms and computation rules, whose operational semantics is given in terms of two relations: simplification and computation. Simplification is induced by confluent rewriting on terms. Computation is induced by chemical reactions, like those in the Join-calculus. The language can serve as metalanguage for defining the operational semantics of other languages. This is demonstrated by defining encodings of several calculi (representing idealized programming languages).