Surface plasmon resonance (SPR) is a suitable technique to optically characterize thin layers. It has often been stated that SPR cannot determine simultaneously both the thickness and the dielectric constant (possibly complex) of the layer. We demonstrate that this idea arises from an error caused by the method used in the simulation of the reflectivity curve. So we do not have to design complex systems to solve this problem as did previous authors. In this paper, we report on a simulation based on the Fresnel equations to calculate the reflectivity curve from the critical angle (?c ) to cover a wider range of ? values. With a matrix formalism, we can pick the layers we want very easily. With our process, results are excellent for all kind of noise distributions even at high noise levels. A metallic layer was first characterized; then an ultrathin (12 Å) dielectric layer was added. We also calculated the standard deviations for all cases to prove that the SPR technique is a very sensitive probe. In all studies, the results showed very good agreement between the true values of the parameters and the simulated ones, as well as small standard deviations.