A new laseroptical measurement technique for the determination of concentrations within liquid/liquid systems - rainbow refractometry - is being introduced. Beside giving an overview of the physical background and the principle of measurement the options of determination of homogeneous equilibrium concentrations within droplets by measuring the position of the primary monochromatic rainbow are explained experimentallly for the system toluene/water/acetone. For the investigation of the effect of concentration gradients induced by instationary mass transfer processes a coupled model of diffusion and refractive index gradient for the rainbow is introduced. Data simulated by this model are being compared to experimentally obtained values and discussed briefly. It is obvious that the slope of the rainbow position can give information about concentration gradients.