This paper reviews mechanistic investigations of the methanol-to-olefin (MTO) process by in situ solid-state NMR spectroscopy, mainly performed under continuous-flow and stopped-flow conditions. During methanol conversion on the silicoaluminophosphate H-SAPO-34 under continuous-flow conditions, a hydrocarbon-pool consisting of a mixture of C-6-C-12 olefins and aromatics was found. The hydrocarbon-pool mechanism was verified as the dominating route in the MTO process under steady-state conditions. By in situ MAS NMR experiments under continuous-flow conditions it could be demonstrated that methanol molecules are added to the hydrocarbon-pool and light olefins are split off as reaction products. The reactivity of surface methoxy groups in the MTO process were evidenced by the reaction with different probe molecules. By the new developed in situ MAS NMR-UV/vis technique, the initial formation of the hydrocarbon-pool during the induction period was investigated. (c) 2005 Elsevier B.V. All rights reserved.