The formation mechanism of large defect cavities (∅ 11-23 nm) beside the MCM41 mesopores (∅ ∼ 2.3 nm), was studied from mixtures of sodium silicate, cetyl trimethyl ammonium bromide (CTAB) and ethyl acetate (EtAc) with the molar composition 1 SiO2; 1 Na2O; x CTAB; y EtAc: 237 H2O (with x = 0.33 or 0.65, y = 3, 6, 9 or 15). The silica flocculation and polycondensation were obtained by decreasing the pH using the hydrolysis of different amounts of EtAc. The reaction was carried out in two steps at 25 and 80 °C. The samples were characterized by SEM, TEM, XRD, nitrogen adsorption and TGA. The CTA/SiO2 molar ratio, which forms complete silica layers covering the micelles in the MCM41 structure, was estimated from the area covered by the CTA cations in the micelles head and the silica tetrahedron area in the layers. The number of silicate anions needed to compensate the CTA cations charge was determined in function of the pH from the anions ionization constants. From these two parameters, a model, showing the percentage of missing tetrahedra in the silica layers, was established in function of the pH and the CTA/SiO2 molar ratio, and used to propose a mechanism which explains the formation of large defect cavities. Their size was discussed taking into account the pH decrease rate, which is function of the EtAc amount. The particles morphology, varying from long and curved prismatic rods to isometric particles of strongly bended short rods, is due to their softness, which results of the presence of the numerous defect cavities.