Using synchrotron X-ray powder diffraction-mass spectrometry in combination with theoretical calculations, we probed the structural changes of adsorbed methanol species in H-ZSM-5 for the first time during catalytic conversion of methanol. Preliminary experimental and computational findings suggested that associative mechanism was the dominant reaction pathway in the early stage of the reaction at 200 degrees C. Also, we observed a minor contribution from dissociative mechanism of the adsorbed methoxy intermediate species, CH3-O-z, (where O-z is a framework oxygen atom), at Bronsted acid sites. It led to methoxy accumulation during the late stage of the reaction, which was postulated to be the initial step that produced 'hydrocarbon pool' and 'coke' through the formation of C-C bonds. (C) 2018 Elsevier Inc. All rights reserved.