In a process for separating one or more 3,3′-, 3,4′- and 4,4′-dimethyl biphenyl isomers, a feed comprising the isomers is contacted with a zeolite adsorbent which contains one or more metal cations in the +1 or +2 oxidation states. Separation processes for each of the 3,3′-, 3,4′- and 4,4′-dimethyl biphenyl isomers is provided.