Method and apparatus for determining dielectric function of liquid solutions and thereby concentrations of substances in aqueous solution or the volatile/non-volatile nature of the liquid by self-referenced reflection THz spectroscopy. Having the aqueous solution in any container with a window allows irradiating coherent THz radiation with frequencies within the range 0.05 - 2 THz on the front of the window, and recording both a reference signal reflected from the front of the window and a sample signal reflected from the back of the window in contact with the aqueous solution. From these signals, the complex index of refraction, (I) or the complex reflection coefficient (II), can be calculated. The calculated components are compared with previously determined components from samples with known concentrations, whereby a concentration of the substance in the mixture can be estimated. The invention is particularly useful for determining alcohol (ethanol) content in aqueous solution containing other substances and particles.