Stepanov and Luzgin recently applied solid-state NMR with magic-angle spinning (MAS) to the reactions of acetonitrile with olefins or alcohols in zeolite HZSM-5 at 296 K and observed signals they assigned to N-alkylnitrilium cations (Chem. fur. J. 1997, 3, 47). Here we report strong verification of their assignment and show that these cations can also form in catalytic flow reactors. We used the recently developed pulse-quench reactor to contact acetonitrile and either tert-butyl alcohol or isobutylene for several seconds at temperatures up to 433 K followed by a rapid thermal quench. C-13 and N-15 MAS NMR measurements were performed at 77-298 K to identify the species present in the zeolite and to permit determination of the principal components of the chemical shift tensors. We optimized the geometry of the N-tert-butylacetonitrilium cation and acetonitrile using density functional theory at B3LYP/DZVP2 and then calculated theoretical chemical shift tensors using the GIAO-MP2 level of theory and two basis set schemes. Essentially quantitative agreement was obtained between theoretical and experimental C-13 principal components for the nitrile carbon tensor, and the agreement for the N-15 tensor was almost equally good.