Abstract: A method of determining surface plasmon resonances at two-dimensional measurement surfaces, in which the measurement surface which is formed by a thin metal film and which is brought into contact with a specimen to be measured is illuminated and the intensity distribution in the reflected beam is measured is to be further developed in such a way that it is substantially more sensitive (in respect of measurement) and is more resistant to interference. That is achieved in that the measurement surface is illuminated with two collimated monochromatic laser beams of differing wavelengths which are spatially combined to form an overall beam and the difference in the intensities reflected at the two wavelengths is measured to characterise plasma resonance.