The rates of ferriin [i.e. tris(1,10-phenanthroline)iron(III)] oxidation of cis- and trans-1,2-diphenylcyclohexane-1,2-diol have been found to be dramatically different; cis-1,2-diphenylcyclohexane-1,2-diol reacts a minimum of 10⁴ times faster than the corresponding trans-isomer; implications for the oxidation of benzylic diols by ferriin are discussed.