The separation of xylene isomers was attempted using the principle of selective guest inclusion into a guanidinium p-toluenesulfonate (GTS) host frame. In a pair-wise competition of xylene isomers, m-xylene was selected most by the host frame over the other isomers. Meanwhile, there was hardly any competition between o-xylene and p-xylene for inclusion in the GTS host frame, resulting in only minimal selectivity in the inclusion compound. Differential scanning calorimetry was used to determine the thermal energy required to release the guest molecules from the inclusion compounds. The C-H... 0 interaction in the inclusion compound of GTS.(m-xylene) provided a stronger host-frame structure than with GTS.(o-xylene) or GTS.(p-xylene). Plus, GTS.(o-xylene) and GTS.(p-xylene) exhibited similar guest-releasing energies, as reflected in the guest selectivity of the GTS host. Thus, m-xylene was the guest template that produced a stronger GTS host-frame structure, and was selected for inclusion more than the other isomers. (c) 2008 Elsevier B.V. All rights reserved.