C-13 cross-polarization magic angle spinning (CP-MAS) NMR evidence for arylmethyl carbocations formed from a series of C-13-labeled precursors adsorbed at room temperature on heat-pretreated zeolite HY is reported and discussed. The reactivities of precursors of three binds of arylmethyl carbocations, (C6H5)(3-) C-13(+), (C6H5)(2)(C+H)-C-13, and (C6H5C+H2)-C-13, are compared, and evidence for the chemical transformations of arylmethylium precursors on zeolite HY is provided by analyzing CDCl3 extracts. High-field (14.1 T) Al-27 MAX NMR spectra are obtained on preheated zeolite HY as a function of the heating temperature under vacuum (10(-3) Torr). An intense peak, which might be related to a low-symmetry AlOn species, is observed with an Al-27 chemical shift of about 23 ppm for a zeolite HY sample that has been heated at 600 degrees C. Several convenient procedures are described for the preparation of C-13-labeled arylmethyl carbocation precursors.