The admittance technique is used to solve Maxwell's equations for an inhomogenous system consisting of a surface liquid film of low-absorbing (insulating) material on a metallic aluminium substrate. The power reflectivity spectrum in σ and π polarisation is easily discernible and is amplified to full scale for most angles of incidence. The reflectivity spectrum is very sensitive to the thickness of the liquid film and to the angle of incidence. This provides scope for using this technique to detect thin films of polymer material on metal substrates, oxide films, interfaces and so on.