A method of analysing a refractive index of an analysed medium by means of a polarisation-sensitive optical sensor comprising an optical prism provided with a dielectric multilayer having a side adjacent to the optical prism and a sensor side adapted to come into contact with the analysed medium, wherein the analysed medium is a mixture comprising 0 to 100 % of component A and 100 to 0 % of component B, wherein the mixture exhibits a change in refractive index depending on the ratio of components A and B, wherein the method comprises the steps of: a. contacting the analysed medium having a refractive index n 1 with the sensor side of the dielectric multilayer; b. illuminating the optical prism and the side of the dielectric multilayer adjacent to the optical prism by a collimated and linearly polarised beam comprising both s- and p-components of a polarised wave generated by a quasi-monochromatic source such that the angle of incidence θ is less than the critical angle θ crit for excitation of a Bloch surface wave, wherein afterwards at least a portion of the s - and p -component of the polarised wave is reflected from the side of the dielectric multilayer adjacent to the optical prism, which is manifested in a reflectance ratio Rp/Rs for the analysed medium having a refractive index n 1, where Rp and Rs are reflectances for the p - and s -components of the polarised wave, wherein the wavelength of the quasi-monochromatic source is selected such that the reflectance ratio Rp/Rs at said wavelength changes to a greater extent as compared to a resonance wavelength for excitation of the Bloch surface wave; c. separating the s - and p -components of the polarised wave of a reflected beam into polarisation components and by means of a polarisation splitter; d. detecting each polarisation component (121, 122) by a photodetector (123, 124) to produce an analysable signal corresponding to the reflectance Rp and Rs; and e. evaluating the signal corresponding to the reflectance Rp and Rs by means of a control unit, wherein the reflectance ratio Rp/Rs is a function of the value of refractive index n 1 of the analysed medium, wherein n 1 depends on the ratio of components A and B in the mixture. a. contacting the analysed medium having a refractive index n 1 with the sensor side of the dielectric multilayer; b. illuminating the optical prism and the side of the dielectric multilayer adjacent to the optical prism by a collimated and linearly polarised beam comprising both s- and p-components of a polarised wave generated by a quasi-monochromatic source such that the angle of incidence θ is less than the critical angle θ crit for excitation of a Bloch surface wave, wherein afterwards at least a portion of the s - and p -component of the polarised wave is reflected from the side of the dielectric multilayer adjacent to the optical prism, which is manifested in a reflectance ratio Rp/Rs for the analysed medium having a refractive index n 1, where Rp and Rs are reflectances for the p - and s -components of the polarised wave, wherein the wavelength of the quasi-monochromatic source is selected such that the reflectance ratio Rp/Rs at said wavelength changes to a greater extent as compared to a resonance wavelength for excitation of the Bloch surface wave; c. separating the s - and p -components of the polarised wave of a reflected beam into polarisation components and by means of a polarisation splitter; d. detecting each polarisation component (121, 122) by a photodetector (123, 124) to produce an analysable signal corresponding to the reflectance Rp and Rs; and e. evaluating the signal corresponding to the reflectance Rp and Rs by means of a control unit, wherein the reflectance ratio Rp/Rs is a function of the value of refractive index n 1 of the analysed medium, wherein n 1 depends on the ratio of components A and B in the mixture.