We present an experimental study on the swelling of polyelectrolyte brushes covalently attached to planar solid surfaces in contact with humid air. Monolayers of poly-N-methyl-[4-vinylpyridinium]iodide (MePVP) with film thicknesses of several hundred nanometers were used in this study. The MePVP brushes were attached to the surfaces of silicon wafers as well as to evaporated silicon oxide films on solid substrates by using self-assembled monolayers of an azo initiator and radical chain polymerization in situ. The film thicknesses of the surface-bound monolayers were measured by optical waveguide spectroscopy (OWS) as a function of the humidity of the environment. The MePVP brushes show strong increases in thickness as well as a strong decrease of the refractive index of the surface-attached layer due to water incorporation caused by the exposure to the humid environment.