The invention relates to a method and apparatus for obtaining and analysing physical properties of a substance. Optical data and acoustical data are obtained for the substance and the data are used to apply a model of the optical/acoustical properties of the substance such that thereby any of the thickness, the density, the refractive index and composite related information such as the content of a certain component in the substance can be determined. If dynamic effects are studied, preferably data of simultaneously performed optical and acoustical measurements are used. An example is the use of data of surface plasmon resonance measurements and surface acoustic wave measurements to determine the water content in solutions of organic material.