The initial steps in the formation of ferrisilicate mesoporous molecular sieves in the presence of sodium or tetramethylammonium silicate, cetyltrimethylammonium bromide and an acid (HF, HCl or HNO3) were followed by X-ray powder diffraction, nuclear magnetic resonance of silicon-29 in the solid state (Si-29-MAS-NMR) and electron spin resonance (ESR). The quality of the mesophases obtained (hexagonal or non-parallel mesopores) and even the nature of the phases (mesophases, layered or amorphous) are highly dependent on three factors: the type and polymerization degree of the silicate anions, the presence of iron (III) ions, and the bonding capacity towards CTA(+) of the other inorganic anions present in the reaction mixture. The acid used controls the relative concentrations of each silicon precursor present in the reaction mixture and through that controls also the concentration of iron effectively incorporated into the framework. For HF and HCl reaction mixtures, Fe(III) is already in tetrahedral geometry in the solid part of the reaction mixture. before aging or hydrothermal treatment, more noticeably for HF derived solids than for HCl ones. For HNO3, no tetrahedral Fe(III) was observed by ESR in the solid part of the reaction mixture, before aging and hydrothermal treatment. (C) 2001 Elsevier Science B.V. All rights reserved.4741700397406