Mullite formation from kaolinite was studied by means of high-temperature in situ powder neutron diffraction by heating from room temperature up to 1370°C. Neutron diffractometry under this non-isothermal conditions is suitable for studying high-temperature reaction kinetics and to identify short-lived species which otherwise might escape detection. Data collected from dynamic techniques (neutron diffraction, DTA, TGA and constant-heating rate sintering) were consistent with data gathered in static mode (conventional X-ray diffraction and TEM). The full process occurs in successive stages: (a) kaolinite dehydroxylation yielding metakaolinite in the ∼400–650°C temperature range, (b) nucleation of mullite in the temperature range ∼980–992 to ∼1121°C (primary mullite) side by side with a crystalline cubic phase (Si-Al spinel) detected in the ∼983–1030°C temperature interval; (c) growth of mullite crystals from ∼1136°C, (d) high (or β) cristobalite crystallization at T>∼1200°C and (e) secondary mullite crystallization at T>∼1300°C. The calculated activation energy for the kaolinite dehydration was 115kJ/mol; for the mullite nucleation was 278kJ/mol and for the growth of mullite process was 87kJ/mol; finally for cristobalite nucleation the calculated apparent activation energy was 481kJ/mol.