Valve for dosing and mixing fluids comprising: - at least one first inlet duct for inletting at least one first fluid to be mixed; - at least one second inlet duct for inletting at least one second fluid to be mixed; - a double chamber for controlling the flows placed in fluid communication with the first inlet duct and with the second inlet duct; - at least one actuator configured to regulate the flow rates of the first fluid to be mixed and of the second fluid to be mixed, which flow in the first inlet duct and in the second inlet duct respectively; - at least one mixing duct where the flows of said first fluid and of said second fluid are placed in fluid communication for mixing said first fluid and said second fluid into a mixture of fluids, placed downstream of the double chamber for controlling the flows; - an electronic control unit for controlling the valve; and - a Brix degree optical sensor, placed downstream of said mixing duct and operatively connected to the electronic control unit, said optical sensor being configured to send a measurement and control signal to said electronic control unit, said electronic control unit being configured to drive said actuator so as to proportionally control the fluid flow rates flowing in the first inlet duct and in the second inlet duct respectively, and, thus, to regulate the mixing ratio of the first fluid and of the second fluid, wherein the optical sensor is made in the shape of a casing provided with an inlet duct for the mixture of the first fluid and the second fluid exiting from the mixing duct, as well as with an outlet duct for the mixture of the first fluid and the second fluid exiting from the optical sensor, and wherein a measurement chamber is interposed between the inlet duct of the optical sensor and the outlet duct of the optical sensor, characterised in that said measurement chamber is provided with a plurality of optical components (34, 36, 38) arranged to perform the optical reading of the Brix degrees directly on the flow of the mixture of fluids passing through said measurement chamber, said measurement chamber being provided with a passive portion, consisting of a pipe that puts in communication the inlet duct for inletting the mixture of fluids and the outlet duct for outletting the mixture of fluids, and with one active portion, comprising a measurement optical prism, an optical signal emitter device and an optical signal receiver device, wherein said measurement optical prism performs the guiding of the wave of the optical signal emitted by the emitter device and received by the receiver device, wherein the active portion of the measurement chamber constitutes a part of the passive portion of the measurement chamber, namely the pipe in which the mixture of fluids flows. - at least one first inlet duct for inletting at least one first fluid to be mixed; - at least one second inlet duct for inletting at least one second fluid to be mixed; - a double chamber for controlling the flows placed in fluid communication with the first inlet duct and with the second inlet duct; - at least one actuator configured to regulate the flow rates of the first fluid to be mixed and of the second fluid to be mixed, which flow in the first inlet duct and in the second inlet duct respectively; - at least one mixing duct where the flows of said first fluid and of said second fluid are placed in fluid communication for mixing said first fluid and said second fluid into a mixture of fluids, placed downstream of the double chamber for controlling the flows; - an electronic control unit for controlling the valve; and - a Brix degree optical sensor, placed downstream of said mixing duct and operatively connected to the electronic control unit, said optical sensor being configured to send a measurement and control signal to said electronic control unit, said electronic control unit being configured to drive said actuator so as to proportionally control the fluid flow rates flowing in the first inlet duct and in the second inlet duct respectively, and, thus, to regulate the mixing ratio of the first fluid and of the second fluid,