A theoretical and experimental study of the diffusion-controlled solidification process of an aqueous solution of ammonium chloride has been performed to obtain fundamental understanding relevant to metal casting, solidification of alloys, and freezing of biological materials. The effective thermal conductivity of the solidifying system is calculated using different models and the model predictions are compared. The model is validated by comparing the predictions with experimental data. It was found that, for the conditions considered in the present study, the parallel model, which is a simple average of the thermal conductivities of the two phases, leads to acceptable results. The reasons for this are related to the size of the mushy region and the morphology of the crystals during the solidification process.