In situ C-13 NMR with both CAVERN and pulse-quench methods was used to understand the chemistry of acetone on the silico-aluminophosphate catalyst HSAPO-34. The isotropic shift of [2-C-13]acetone shows that most of the acid sites on this catalyst are weaker than those on aluminosilicate zeolites, but a minority site was resolved with a shift higher than that on zeolite HZSM-5. At elevated temperatures, acetone dimerized to diacetone alcohol and dehydrated to mesityl oxide. Mesityl oxide cracked in the presence of water to acetic acid and isobutylene. Trimerization of butenes formed a significant amount of a persistent carbenium ion with unusual spectroscopic properties, and this is proposed to be the heptamethylcyclopentenyl cation, the first observation of a persistent carbenium ion on a SAPO catalyst. Chemical shift calculations at the GIAO-MP2 level revealed a discrepancy for one signal of the proposed cation but ruled out an alternative assignment. Methylaromatic formation coincided with cyclopentenyl cation synthesis, supporting a mechanistic proposal for aromatic synthesis on zeolites. Conventional flow reactor studies revealed a high selectivity for C-4 olefins and rapid deactivation with acetone as feed. The results of this investigation are interpreted in the context of methanol-to-olefin (MTO) chemistry on HSAPO-34.