Diphasic gels of four different compositions were synthesized. Lattice parameters, crystallite size, and strain values of mullite formed at different stages of annealing of diphasic gels have been calculated by quantitative standard less analysis using Xray diffraction. Results show that all three parameters change on progressive heating. Changes in the microstrain of multite show a reciprocal relationship with crystallite size. Silica-rich gel shows that the relative changes in the calculated values of three structural parameters of mullite formed during its formation process are because of an increase in its crystallinity rather than changes in composition. However, gels of composition in the solid solution range show that the relationship between lattice constants and chemical composition of mullite formed at high temperature agrees with Cameron data.