A method for in-line analysis of a composite product that advances in a machine for producing sanitary articles, comprising a preliminary step and an operating step, wherein said preliminary step comprises: - arranging two samples of target materials (14, 16), which are part of said composite product, composed of cellulose fluff and granular superabsorbent polymer, respectively; - carrying out a spectral analysis of said two samples of target materials (14, 16); - determining spectral reflectance or transmittance data (R14, R16) of each of said target materials (14, 16), - identifying a reference wavelength (Lr) at which the difference (dR) between the spectral reflectances or transmittances (R14, R16) of said at least two target materials (14, 16) is maximum; wherein said reference wavelength (Lr) is between 1300 nm and 1500 nm; and wherein said operating step comprises: - advancing said composite product in a feed direction in a machine for producing absorbent sanitary products, at a speed from 100 to 800 m/1', - illuminating said composite product with electromagnetic radiation having an emission range including said reference wavelength (Lr), - receiving electromagnetic radiation reflected by, or transmitted through, said composite product by means of an optical sensor sensitive to said reference wavelength (Lr), - processing the signals provided by said optical sensor, and obtaining information on characteristic parameters of said two target materials (14, 16) contained in the composite product. wherein said preliminary step comprises: - arranging two samples of target materials (14, 16), which are part of said composite product, composed of cellulose fluff and granular superabsorbent polymer, respectively; - carrying out a spectral analysis of said two samples of target materials (14, 16); - determining spectral reflectance or transmittance data (R14, R16) of each of said target materials (14, 16), - identifying a reference wavelength (Lr) at which the difference (dR) between the spectral reflectances or transmittances (R14, R16) of said at least two target materials (14, 16) is maximum; wherein said reference wavelength (Lr) is between 1300 nm and 1500 nm; - arranging two samples of target materials (14, 16), which are part of said composite product, composed of cellulose fluff and granular superabsorbent polymer, respectively; - carrying out a spectral analysis of said two samples of target materials (14, 16); - determining spectral reflectance or transmittance data (R14, R16) of each of said target materials (14, 16), - identifying a reference wavelength (Lr) at which the difference (dR) between the spectral reflectances or transmittances (R14, R16) of said at least two target materials (14, 16) is maximum; wherein said reference wavelength (Lr) is between 1300 nm and 1500 nm; and wherein said operating step comprises: - advancing said composite product in a feed direction in a machine for producing absorbent sanitary products, at a speed from 100 to 800 m/1', - illuminating said composite product with electromagnetic radiation having an emission range including said reference wavelength (Lr), - receiving electromagnetic radiation reflected by, or transmitted through, said composite product by means of an optical sensor sensitive to said reference wavelength (Lr), - processing the signals provided by said optical sensor, and obtaining information on characteristic parameters of said two target materials (14, 16) contained in the composite product. - advancing said composite product in a feed direction in a machine for producing absorbent sanitary products, at a speed from 100 to 800 m/1', - illuminating said composite product with electromagnetic radiation having an emission range including said reference wavelength (Lr), - receiving electromagnetic radiation reflected by, or transmitted through, said composite product by means of an optical sensor sensitive to said reference wavelength (Lr), - processing the signals provided by said optical sensor, and obtaining information on characteristic parameters of said two target materials (14, 16) contained in the composite product.