Liquid phase diffusion is a high energy consumption process in chemical production. With the rapid development of the chemical technology based on liquid phase diffusion, especially in some emerging fields such as biomedicine and environmental science, the microcosmic study on the liquid phase diffusion is becoming more and more important. By applying the digital holographic interferometry, a diffusion process in liquid phase of pure ethanol and water was observed. After recording the digital hologram by a Mach-Zehnder interferometer and reconstructing digital hologram of object beam by the gray scale analysis, the phase variation of object wavefront in the diffusion process at different times and the curves of refractive index change in the diffusion area were both acquired. The result shows that by applying the digital holographic interferometry, a fast, real time and high-accuracy diffusion can be realized. Moreover, this method can be used to realize a remote visual monitoring. In addition, this method combining with wavelength and angular multiplexing techniques can be applied to obtain nonlinear characteristics parameters in multiple-phase diffusion measurements.