Via conversion of C-13-enriched methanol and applying a new stopped-flow protocol, the Bronsted acid sites (SiOHAl groups) of a calcined zeolite H-Y were highly covered by surface methoxy groups (SiOCH3Al). These methoxy groups delta(13C) = 56.6 ppm) were formed at bridging OH groups in the direct vicinity of framework aluminum atoms. The methylated zeolite Y was used to perform first Al-27 spin-echo NMR investigations of the influence of surface methoxy groups on zeolitic framework aluminum atoms. The formation of methoxy groups on calcined zeolite H-Y was found to decrease the Al-27 quadrupole coupling constant of framework aluminum atoms from 14.8 +/- 0.2 MHz (uncovered SiOHAl groups) to 12.8 +/- 0.2 MHz (SiOCH3Al). (C) Elsevier Science B.V. All rights reserved.