The use of optical waveguides as chemical sensors requires a referencing system to compensate for the effect of contamination on the surface of the sensor. In this paper, we propose the use of two waves with different penetration depths for their evanescent fields in order to correct measurements. Several schemes are considered, including two-wavelength, two-mode and polarisation discrimination techniques. It is shown that good results can be obtained using TE0 and TM0 modes in a high index film.