Measuring device (1; 15) for determining the composition of the liquid phase of a liquid-gas mixture includes a duct (2) defining a flow direction (X) of the mixture parallel to the longitudinal development axis (Y) of the duct (2) and a measuring element (4) arranged in the duct (2) and suited to determine the composition of a liquid layer that flows in contact with the internal surface (3) of the duct (2). The internal surface (3) of the duct (2) includes an intercepting surface (5, 5′) suited to convey part of the liquid layer towards the measuring element (4), arranged so that it is incident on the flow direction (X) and developed according to a conveyance trajectory that has a helical section and whose tangent to the outlet end (7, 7′) intersects the measuring element (4).