We are studying semantics of a small object-based language, with the following main characteristics: parallelism, dynamicity, high order parameters, notion of a global instant, and reactivity. We give formal semantics using two related formalisms , namely $\pi$-calculus and the so-called «Chemical Abstract Machine» (\Name{CHAM}). These formalisms are both powerful enough to express dynamicity and high order parameters, but they give distinct insights into the language. In $\pi$-calculus, emphasis is put on communications through channels; on the other hand, in \Name{CHAM}, emphasis is put on more abstract and general interactions between program parts. We finally prove adequacy of the $\pi$-calculus semantics w.r.t. the \Name{CHA- M} semantics.