Flavianic acid disodium salt (naphthol yellow S), which is used as yellow colorant for cosmetics of dyeing hair, can decompose and burn spontaneously in the producing process. The thermal stability of flavianic acid disodium salt was investigated theoretically and experimentally in this paper. The prediction results by the Computer Program for Chemical Thermodynamic and Energy Release Evaluation (CHETAH) suggest flavianic acid hydrate (the former production of flavianic acid disodium salt) and flavianic acid disodium salt have high rank of maximum heat of decomposition and fuel value-heat of decomposition The rank of explosive hazard for flavianic acid disodium salt was II by the Differential Scanning Calorimetry (DSC) and Pressure Vessel Test (PVT) according to the evaluation method in the Japanese Fire Service Law. The experimental results by the Accelerating Rate Calorimeter (ARC) demonstrate particle size difference has no significant effect on the thermal stability of flavianic acid disodium salt Compared with flavianic acid disodium salt, flavianic acid hydrate had low thermal stability. The results of the C 80 confirmed these results further. It suggests that the thermal decomposition would occur if flavianic acid hydrate was remained in the final product of flavianic acid disodium salt and the temperature can reach 150 degrees C when excess heat occurred in the grinding process. (C) 2009 Elsevier Ltd All rights reserved.