Experimental evidence for the presence of tert-butyl cations, which are important intermediates in acid-catalyzed heterogeneous reactions, on solid acids has still not been provided to date. By combining density functional theory (DFT) calculations with 1H/13C magic-angle-spinning NMR spectroscopy, the tert-butyl cation was successfully identified on zeolite H-ZSM-5 upon conversion of isobutene by capturing this intermediate with ammonia. Captured and characterized: The intermediacy of tert-butyl cations in the acid-catalyzed conversion of isobutene on zeolite H-ZSM-5 was confirmed by their capture with ammonia.