In this study, differential scanning calorimetry (DSC) has been applied to measure the kinetics of nonisothermal crystallization of potassium nitrate and ammonium perchlorate from unsaturated and saturated aqueous solutions. DSC data have been analyzed by an advanced isoconversional method that demonstrates that the process is represented by negative values of the effective activation energy, which varies with the progress of crystallization. The classical nucleation model can be used to predict and understand the experimentally observed variation in the effective activation energy. The saturated and unsaturated solutions have demonstrated distinctly different crystallization kinetics. It is suggested that the unsaturated solutions undergo a change in crystallization mechanism from homogeneous to heterogeneous nucleation.