A simple completeness proof for the axiomatisations of weak behavioural equivalences Yuxin Deng This paper presents a simple but uniform completeness proof for the axiomatisations of five weak behavioural equivalences: branching congruence, $\eta$-congruence, delay congruence, quasi-branching congruence, and weak congruence in the basic CCS without recursion. For the first three congruences, our result improves van Glabbeek and Weijland's completeness proof of using graph rewriting by a more direct proof that is fully equational. For quasi-branching congruence, our completeness result is, to the best of our knowledge, new.