A Mach-Zehnder interferometer has been developed for the monitoring of the kinetics of the diffusion process in protein crystal growth. This device can be used in conjunction with the ESA Advanced Protein Crystallization Facility (APCF), which allows experiments under microgravity conditions (e.g. on board the NASA Space Shuttle). Experimental trials on the ground have been carried out with the interferometer using the engineering model of the APCF and a protein dialysis reactor. Chicken egg-white lysozyme crystal growth, as a test, has thereby been monitored directly. The changes of concentration in the solution over time have been determined via the refractive index measurements made and subsequently correlated with visual monitoring of crystal growth in a repeat experiment.