Variable-temperature magnetic susceptibility measurements performed on two hexairon(III) clusters containing a mu6-oxo bridge show that this unusual bridging mode leads to weak antiferromagnetic exchange-coupling interactions between the metal centers, with J(trans) and J(cis) values of 19(2) and 9(1) cm-1, respectively. The result is relevant to the search for magneto-structural correlations in oxo-bridged polyiron aggregates.