Experiments on fast transient mass diffusion between a NaCl solution and pure water were conducted. The speed of mass propagation, the concentration profiles, and other quantities were determined experimentally. A physical mechanism that is different from the description of classical diffusion law was revealed by the experiments, and a related theoretical analysis was also made. The results indicate that the calculated results are in good agreement with the experimental data. The modified Ficks formulation more realistically reflects the characteristics of fast transient mass diffusion.