The coupled simulation equivalence is slightly larger than observation equivalence. Where observation equivalence is based on weak bisimulations, coupled simulation equivalence is based on pairs of simulations which coincide at stable states. We establish the corresponding congruence and provide a complete axiomatization by adding a new -law, .(. P + Q) = . P + Q, to the axiomatization of observation congruence. We further indicate how the definition of the equivalence can be extended to divergent transition systems.