The object of the present invention relates to a system for measuring the salinity of hydrocarbons at the bottom of an oil well, using the technique of time domain reflectometry (TDR). The system comprises an electromagnetic pulse generator, an oscilloscope for displaying and measuring the frequency, amplitude and wavelength of the signal, a signal amplifier, a computer for processing and storing the information, and a metal wire that functions as a waveguide To transmit the signal from the signal generator to the hydrocarbon to which its salinity is to be determined at the bottom of the well. The signal returns from the bottom of the well to the oscilloscope where the difference between the sent signal and the return signal is measured. This difference allows us to infer the salinity of the hydrocarbon. The guide wire is attached to the production line by means of a strap or other fastening device from the surface to the bottom of the well, where the tip of the cable is inserted into the pipe to contact the hydrocarbon and in this way detect its salinity. It is possible to use the same pipe as a waveguide to transmit the test signal to the bottom of the well. In addition, the salinity of the hydrocarbon can be determined at different points along the well.