The interfacial interaction of Mo species with the HBeta zeolite was studied by multinuclear MAS NMR, XRD and N-2 adsorption. As proved by the quantitative Al-27 MAS NMR, this interaction is so strong as to dealuminate the framework of HBeta, and leads to a new peak appearing at -14 ppm, which indicates the formation of crystalline Al-2(MoO4)(3). This can also be detected by XRD measurements when the Mo loading is as high as 9.0 wt.%. The corresponding quantitative Si-29 and H-1 MAS NMR spectra show that the amount of silanols and Bronsted acidic sites decrease obviously with increasing Mo loading. This also reveals an interaction between Mo species and HBeta support through an oxygen bridge resulting from condensation with the hydroxyls on the support. At higher Mo loadings, the interaction is so strong that it results in an extraction of aluminum from the zeolite framework, and subsequently appearance of Al-2(MoO4)(3) and loss of Bronsted acidic sites. These can be correlated to the low catalytic activity of Mo/HBeta in metathesis of ethylene and 2-butylene to propylene. (c) 2006 Elsevier B.V. All rights reserved.