A fluorescent aryl azide [N-methyl-N-(2-hydroxyethyl)-5-(dimethylamino)naphthalene-1-sulfonamide 4-azidobenzoate] (2) was synthesized as a model compound for aryl azides that are used for the photochemical surface modification of poly(ether urethanes) (PEUs). We are using this method of surface modification in our search for artificial surfaces with optimized blood compatibility. As a part of this work, two pivotal questions are now addressed : (i) what is the surface density of coupled molecules that can be reached by this method and (ii) is binding to the surface really covalent? After the photochemical coupling of 2 to a PEU surface, the ester bonds of the coupled molecules were selectively hydrolyzed with 0.1 M NaOMe/MeOH and the concentration of fluorescent molecules in solution determined spectrofluorometrically. Since not all of the fluorescent molecules were removed by this procedure, an additional experiment was performed (in which the polymer was completely dissolved). It was found that 25 nmol/cm(2) had coupled to the outermost polymer layers and 7.7 nmol/cm(2) to polymer layers more within the material (unreachable for the NaOMe/MeOH reagent). In another experiment a modified PEU sheet was dissolved in THF, after which MeOH was added to precipitate it again. From the fact that no fluorescence was detected in solution it could be concluded that all of the fluorescent molecules were indeed covalently coupled to the polymer surface.