Abstract The microstructural evolution of an illito-kaolinitic raw material has been characterized during firing at 10 °C/min up to 1200 °C. The strong evolution of porosity and amount of mullite formed from the viscous flux (from 1000 °C) has been quantified using dilatometric measurement, image analysis and X-ray diffraction. In situ ultrasonic echography has been used in order to determine the Young's modulus ( E) evolution associated to the microstructural changes. This technique is highly sensitive to porosity elimination and mullite development even though an abundant viscous flux is present. For low amount of mullite (<24.7 vol.%), the E evolution observed can be easily related to porosity and mullite volume proportion changes by applying the Hashin & Shtrikman approach (lower bound). For higher mullite content, the strong experimental E increase observed between 25.9 and 51.2 GPa has been related to the transition from isolated rigid inclusions (mullite and quartz) in soft matrix (viscous flux) toward a percolating network.