Mesoporous MCM-41 materials with a distinct N-2-sorption hysteresis behavior have been prepared from pure silica and aluminosilicate-C-16 trimethylammonium (TMA)Br systems by a delayed neutralization procedure. On the basis of the analysis of transmission electron microscopy micrographs of these MCM-41 materials, we observed that the sample with large type-H4 hysteresis loop at p/p(0) between 0.5 and 1.0 contains extensive structural defect holes amid the nanochannels. These holes are irregular in shape and their size distributes between 5.0 and 30.0 nm. The pore-blocking effect leads to the hysteresis in desorption. Aluminosilicate MCM-41 often possesses a larger hysteresis loop than pure silica MCM-41. The linear channel system of MCM-41 becomes effectively interconnected through these defect holes. The unusual adsorption hysteresis is associated with the pore-blocking effect around the embedded voids in the framework structures. The size of the adsorption-desorption hysteresis loop is proportional to the volume of hole defects in the nanochannels, and it is dependent on the synthesis conditions such as water content, Si/Al ratio, and morphology. Tubular morphology is often associated with large hysteresis behavior and thus more hole defects. The interconnecting channels through defect holes thus makes the diffusion of molecules inside the MCM-41 structure more effective, which is important in catalysis applications.