A method of measuring a refractive index of a liquid substance under test with a time-domain spectroscopy measurement system adapted to measuring a propagation of radiation pulse through a container, including a step of determination of a difference between a propagation time determined in a base measurement step, and a propagation time, determined in a main measurement step, through the container comprising the liquid substance under test, and computing the refractive index of the substance under test based on said difference, wherein the radiation pulse is a terahertz radiation pulse, characterized in that before the main measurement step the liquid substance under test is diluted with a known solvent of an absorption lower than absorption of the liquid substance under test, to obtain a solution having absorption low enough to enable a measurement of the terahertz radiation pulse transmitted through the container filled with the solution, and in the main measurement step a first concentration of the substance under test in the solution is recorded, and a first time of propagation of the terahertz radiation pulse in the solution is measured, then the substance under test is diluted to a second concentration and the main measurement step is repeated so that a second concentration of the test liquid substance in the solution is recorded, and a second time of propagation in the solution is measured, wherein the base measurement is carried out with the container filled with the pure solvent to obtain a base propagation time, and then a mixing rule for modelling the dilution is selected so as to fit the first concentration and the first propagation time and the second concentration and the second propagation time and the base propagation time, and based on the selected mixing rule and known refractive index of the solvent, the refractive index of the liquid substance under test is determined based on the difference between the base propagation time and at least one of the propagation times determined in the main measurement. a propagation time determined in a base measurement step, and a propagation time, determined in a main measurement step, through the container comprising the liquid substance under test, and computing the refractive index of the substance under test based on said difference, wherein the radiation pulse is a terahertz radiation pulse, characterized in that before the main measurement step the liquid substance under test is diluted with a known solvent of an absorption lower than absorption of the liquid substance under test, to obtain a solution having absorption low enough to enable a measurement of the terahertz radiation pulse transmitted through the container filled with the solution, and in the main measurement step a first concentration of the substance under test in the solution is recorded, and a first time of propagation of the terahertz radiation pulse in the solution is measured, a first concentration of the substance under test in the solution is recorded, and a first time of propagation of the terahertz radiation pulse in the solution is measured, then the substance under test is diluted to a second concentration and the main measurement step is repeated so that a second concentration of the test liquid substance in the solution is recorded, and a second time of propagation in the solution is measured, a second concentration of the test liquid substance in the solution is recorded, and a second time of propagation in the solution is measured, wherein the base measurement is carried out with the container filled with the pure solvent to obtain a base propagation time, and then a mixing rule for modelling the dilution is selected so as to fit the first concentration and the first propagation time and the second concentration and the second propagation time and the base propagation time, and based on the selected mixing rule and known refractive index of the solvent, the refractive index of the liquid substance under test is determined based on the difference between the base propagation time and at least one of the propagation times determined in the main measurement.