Term rewriting in the presence of associative and commutative function symbols constitutes a highly expressive model of computation, which is for example well-suited to reason about parallel computations. However, it is well-known that the standard notion of termination does not apply any more: any term rewrite system containing a commutativity rule is nonterminating. Thus, instead of adding AC-rules to a rewrite system and trying to prove termination, we switch to the notion of AC-termination. AC-termination can for example be shown using well-founded AC-compatible reduction orders. One specific example of such an order is ACKBO. We present our Isabelle/HOL formalization of the ACKBO order. On an abstract level this gives us a mechanized proof of the fact that ACKBO is indeed an AC-compatible reduction order. Moreover, we integrated corresponding check functions into the verified certifier CeTA. This has the more practical consequence of enabling the machine certification of AC-termination proofs generated by automated termination tools.