MCM-41 aluminosilicate mesostructures (Si/Al = 18) with hexagonal long-range order were assembled from protozeolitic nanoclusters (zeolite seeds) that normally nucleate the crystn. of b-zeolite (BEA). Depending on the duration of the thermal treatment leading to the zeolite seeds, different acidic materials can be obtained. For short pre-treatment duration, the material presents the main characteristics of the MCM-41, whereas for longer pre-treatment duration, there is a gradual transition to a mostly zeolitic one. Hydrocracking of n-decane was carried out on catalysts composed of a sulfided nickel tungsten phase supported on various b-MCM-41 supports as well as on an amorphous silica-alumina and an AlSBA-15 material. The hydrocracking properties of the catalysts are related to the nature of the b-MCM-41 and vary from the comportment of silica-alumina materials to the one of a zeolite. The nickel-tungsten catalyst supported on b-MCM-6 (6 h pre-treatment for the zeolite seeds soln.) is more active and more selective toward isomerization than the catalyst supported on a com. silica-alumina support.