Device for detecting the presence of gas hydrates or of ice in a medium containing water capable of forming solid crystals, characterized in that it comprises at least: - one laser source placed to irradiate at least one measurement point in said medium, - at least two bandpass filters (F1, F2) of preset characteristics, for extracting the light intensities corresponding to the Raman spectra of two vibrational modes of OH bonds, said filters being placed to intercept the signal reflected by said measurement point, said two vibrational modes of OH bonds having wave numbers located at 3160 cm -1 ± 40 cm -1. and at 3400 cm -1 ± 150 cm -1. respectively, - at least one APD detector (D1, D2) for recording the two signals filtered by said bandpass filters (F1, F2), - at least one means for measuring the temperature at said measurement point, - means for analysing whether hydrates are present or ice is present at said measurement point, said analysing means exploiting two characteristic values of said filtered signals, these values being combined with said temperature measurement to determine whether or not ice or hydrate crystals are present, said analysing means being configured to: - calculate a ratio T of said two characteristic values; - make a comparison between said ratio T and limiting values T 0 determined by calibration; - determine whether hydrate crystals are present or ice is present on the basis of said comparison and a comparison between the temperature T measured at said measurement point by said measuring means and a temperature of formation of ice under the measurement conditions. - one laser source placed to irradiate at least one measurement point in said medium, - at least two bandpass filters (F1, F2) of preset characteristics, for extracting the light intensities corresponding to the Raman spectra of two vibrational modes of OH bonds, said filters being placed to intercept the signal reflected by said measurement point, said two vibrational modes of OH bonds having wave numbers located at 3160 cm -1 ± 40 cm -1. and at 3400 cm -1 ± 150 cm -1. respectively, - at least one APD detector (D1, D2) for recording the two signals filtered by said bandpass filters (F1, F2), - at least one means for measuring the temperature at said measurement point, - means for analysing whether hydrates are present or ice is present at said measurement point, said analysing means exploiting two characteristic values of said filtered signals, these values being combined with said temperature measurement to determine whether or not ice or hydrate crystals are present, said analysing means being configured to: - calculate a ratio T of said two characteristic values; - make a comparison between said ratio T and limiting values T 0 determined by calibration; - determine whether hydrate crystals are present or ice is present on the basis of said comparison and a comparison between the temperature T measured at said measurement point by said measuring means and a temperature of formation of ice under the measurement conditions. - calculate a ratio T of said two characteristic values; - make a comparison between said ratio T and limiting values T 0 determined by calibration; - determine whether hydrate crystals are present or ice is present on the basis of said comparison and a comparison between the temperature T measured at said measurement point by said measuring means and a temperature of formation of ice under the measurement conditions.