Device for distinguishing the presence of a liquid phase from the presence of gas hydrate crystals formed from a fluid susceptible to forming gas hydrate crystals, said device comprising: A. a polychromatic first light source capable of emitting radiation at least in the visible domain of the spectrum; B. a body (3, 3', 3") at least a portion of which is in contact with said fluid, said portion of said body (3, 3', 3") comprising a reflective surface having a predefined colour, said predefined colour being associated with a range of wavelengths in the visible domain of the spectrum; C. first means for measuring a light intensity as a function of wavelength and arranged so as to detect radiation re-emitted non-specularly by said reflective surface of said portion of said body (3, 3', 3") having a predefined colour when said reflective surface of said portion of said body (3, 3', 3") having a predefined colour is illuminated by said polychromatic light source; D. a monochromatic second light source emitting in said range of wavelengths which is associated with said predefined colour of said reflective surface of said portion of said body (3, 3', 3"), and E. second means (40, 40') for measuring a light intensity as a function of wavelength and comprising a photodetector (40, 40') sensitive to said range of wavelengths associated with said predefined colour of said reflective surface of said portion of said body (3, 3', 3") and arranged so as to detect radiation resulting from emission by said monochromatic light source and having been specularly reflected off said reflective surface of said portion of said body (3, 3', 3"); F. means for detecting at least said presence of hydrate crystals from said light intensity measured as a function of wavelength by said first means for measuring a light intensity as a function of wavelength, and at least a first predefined threshold, said means for detecting at least said presence of hydrate crystals further making it possible to detect the presence of a liquid phase by also analysing said light intensity measured as a function of wavelength by said second means (40, 40') for measuring a light intensity as a function of wavelength. A. a polychromatic first light source capable of emitting radiation at least in the visible domain of the spectrum; B. a body (3, 3', 3") at least a portion of which is in contact with said fluid, said portion of said body (3, 3', 3") comprising a reflective surface having a predefined colour, said predefined colour being associated with a range of wavelengths in the visible domain of the spectrum; C. first means for measuring a light intensity as a function of wavelength and arranged so as to detect radiation re-emitted non-specularly by said reflective surface of said portion of said body (3, 3', 3") having a predefined colour when said reflective surface of said portion of said body (3, 3', 3") having a predefined colour is illuminated by said polychromatic light source; D. a monochromatic second light source emitting in said range of wavelengths which is associated with said predefined colour of said reflective surface of said portion of said body (3, 3', 3"), and E. second means (40, 40') for measuring a light intensity as a function of wavelength and comprising a photodetector (40, 40') sensitive to said range of wavelengths associated with said predefined colour of said reflective surface of said portion of said body (3, 3', 3") and arranged so as to detect radiation resulting from emission by said monochromatic light source and having been specularly reflected off said reflective surface of said portion of said body (3, 3', 3"); F. means for detecting at least said presence of hydrate crystals from said light intensity measured as a function of wavelength by said first means for measuring a light intensity as a function of wavelength, and at least a first predefined threshold, said means for detecting at least said presence of hydrate crystals further making it possible to detect the presence of a liquid phase by also analysing said light intensity measured as a function of wavelength by said second means (40, 40') for measuring a light intensity as a function of wavelength.