A computer program has been created to separate overlapping exothermic DTA (DSC) effects, obtained under isothermal conditions, occurring due to simultaneous crystallization processes, and also to calculate the kinetic parameters of the processes represented by each peak. A graphical method for the estimation of the required parameters, i.e. initialization, and the refinement of the initial values by the least squares method is presented. The procedure for the determination of the kinetic parameters from DTA data is demonstrated on a model system and on a kinetic analysis of mullite formation from single phase gel. The two-step mullite crystallization model was proposed and the fitting was remarkably good throughout the whole temperature range.