In situ C-13 MAS-NMR was used to study the reactions of [alpha-C-13]styrene, [beta-C-13]styrene, and [alpha-C-13]-alpha-methylstyrene on the acidic zeolites HZSM-5, HY, and H-mordenite. Neither the methylphenylcarbenium ion nor the dimethylphenylcarbenium ion were observed as persistent species by NMR, even at low temperatures. However, styrene reacted on zeolite HY to form an appreciable yield of the 3-methyl-1-phenylindanyl cation, and this cation was stable up to ca, 323 K. The mechanism by which this cation formed was apparently hydride transfer from neutral 1-methyl-3-phenylindan to transient (i.e., not observed) methylphenylcarbenium ion. The analogous 3,3-dimethyl-1-phenylindanyl cation did not form as a persistent species when alpha-methylstyrene was adsorbed on HY; we attribute this negative result to the relative difficulty of intermolecular methyl transfer vs hydride transfer. Styrene dimers cracked on HZSM-5 at ca. 433 K to form a significant yield of the 1-methylindanyl cation, which was stable indefinitely in the zeolite at 298 K. Further heating converted this cation into naphthalene, the ultimate product of dimerization, cracking, skeletal isomerization, and dehydrogenation. A very low yield of the analogous 1,3,3-trimethylindanyl cation formed in the reactions of alpha-methylstyrene on HZSM-5; we attribute the disparity in yields between the less bulky and more bulky cation to the steric effect of the pore size. The yields of both methylindanyl cations on zeolite HY were very low or negligible. This observation may reflect the fact that HZSM-5 is a slightly stronger acid than HY. The above results are introduced and discussed in the context of our recently proposed reclassification of typical zeolites aas strong acids rather than superacids. We also show that the observation of indanyl cations but not simple phenylcarbenium ions as persistent species in high concentration is an expected consequence of the relative acid strengths of the zeolites and carbenium ions. The principal components of the C-13 chemical shift tensor of the charged carbon of the 3-methyl-1-phenylindanyl cation were delta(11) = 310, delta(22) = 286, and delta(33) = 68 ppm; for the 1-methylindanyl cation these values were delta(11) = 359, delta(22) = 320, and delta(33) = 76 ppm.