Self-consistent approaches for quantification of the phase composition of ordered mesoporous materials (OMMs) are proposed on the basis of powder X-ray diffraction (XRD), and, for the first time, on the basis of gas adsorption and thermogravimetry (TGA). A series of hexagonal MCM-41 samples containing different amounts of the lamellar phase was synthesized and characterized using XRD, nitrogen adsorption, and high-resolution TGA. To quantify the phase composition of these samples, purr hexagonal and lamellar materials with appropriate structural properties were prepared, and used to calibrate the quantitative XRD analysis and to develop a new methodology for the phase-composition determination on the basis of gas adsorption and TGA. The phase-composition determination from gas adsorption data was based on the mutual independence of adsorption on different components of mixtures. The relative amounts of components of the mixed phases were obtained by fitting the adsorption isotherms measured on the calcined mixed phase materials with those measured on calcined pure hexagonal and lamellar phases. The resulting fits were remarkably good in the entire pressure range on both adsorption and desorption branches of the isotherms. The TGA quantification of the lamellar phase content was possible because of pronounced differences in the weight loss temperature ranges for hexagonal and lamellar components of the mixed phases. The results of the three methods proposed herein were in a good agreement with one another, and thus these methods were validated for independent use. Moreover, from their very nature, these methods promise to be applicable for determination of the phase composition of other mixed OMM phases.