The temperature dependent effective catalytic pore size can be determined by the comparison of (13)(C) MAS NMR spectra of heterogeneously catalyzed, shape selective reactions and data collected from GC analysis of the product stream of reactions under similar reaction conditions. The concept of effective minimum molecular dimensions has been set forth in a previous paper. (Webster, C. E.: Drago, R. S.; Zerner, M. C. J. Am. Chem. Sue. 1998, 120, 5509-5516.) In this investigation, it is concluded that the effective catalytic channel size of HZSM-5 is between 6.62 and 7.27 Angstrom at 300 degrees C, the MIN-2 dimensions of p-xylene and o-xylene, respectively. At 370 degrees C, the effective catalytic channel size is increased to a minimum of 7.64 Angstrom, the MIN-2 dimension of 1,2,3-trimethylbenzene. We discuss the apparent increase in the effective catalytic channel intersection size with increasing temperature. This novel approach presents an a priori description of relative pore sizes and can be used to investigate and describe numerous catalytic and adsorbent systems as discussed in this paper. The comparison of observed isomer distributions of the trimethylbenzenes to their thermodynamic equilibrium distributions provides experimental evidence for the existence of the previously reported nest effect, which describes the shape selectivity of the exterior surface of the HZSM-5 structure.