Ellipsometry is currently one of the most important techniques for characterization of the deposition and growth mode of ultra thin organic films. However, it is well known that for thicknesses normally encountered in organic monolayer films, as would occur for example in self-assembled monolayers, ellipsometry cannot be used to simultaneously determine the thickness and refractive index of the monolayer film. Current practice is to assume a reasonable value for the film refractive index and calculate an effective 'ellipsometric thickness'. This communication seeks to show that the alternative approach of assuming a thickness for the monolayer (determined by the length of the molecule) and calculating the effective film refractive index lends itself to easier and more meaningful physical interpretation. The Lorentz-Lorenz formula is then used to transform the effective refractive index into a surface coverage and hence to an effective mass coverage. The methodology advanced is applied to the kinetics of formation of a self-assembled monolayer of a well-studied molecule, octadecanethiol on Au.