A Mach-Zehnder interferometer (MZI) was built and modiied to in situ monitor the deformation of polymers during the photocuring process. In this review, the working principle and method of operation of this MZI were explained together with the method of data analysis. As the examples for the utilization of this modiied MZI, measurements of the deformation induced by photopolymerization was demonstrated for three diferent types of samples: homopolymer in the bulk state, miscible polymer blends and phaseseparated polymer blends. Finally, a concluding remark is provided for the usage of MZI in polymer research.