Subwavelength gratings are gratings with periods smaller than the incident wavelength. They possess form birefringence, which depends on the grating parameters. This paper presents the results of an experimental method designed to measure the birefringent properties of diamond subwavelength gratings in the mid-infrared. The method consists of monitoring the intensity transmitted through one polarizer, a subwavelength grating, and a second polarizer for various orientations of the first polarizer. By fitting the intensity variation as a function of the orientation of the first polarizer, one can compute the phase shift induced by the grating, its local fast axis orientation, and the ratio of the transverse electric and transverse magnetic transmission efficiencies. The paper describes the method principle and its mathematical model. Then, several numerical simulations of different subwavelength gratings are presented and their results are discussed. Finally, the optical setup is described and the measurements of one subwavelength grating are displayed and compared with the values expected from the manufacturing process.