Denitrification kinetics of a synthetic substrate containing molasses, sodium nitrate, Na2HPO4 . 12H(2)O,KH2PO4 and tap water treated with Na2SO3 was studied in a rotating disk biofilm reactor (RDBR). Experiments were performed at various conditions: biofilm thickness in the range 100-1100 mu m and nitrate concentration in the feed of 5 to 50 mg N1(-1). Biofilm density decreases with increasing biofilm thickness. Most experiments were in the kinetic controlled regime or fully penetrated biofilms. The intrinsic zero-order kinetic constant was found to be k(0) = 7.96 x 10(-6) kg NO3-N*kg biofilm(-1) s(-1) (at 25 degrees C). Some runs were in the diffusion controlled regime or partially penetrated biofilms; the effective diffusivity of nitrate in the biofilm which fitted experimental results was D-b = 1.5 x 10(-9) m(2) s(-1). The efficiency of the RDBR, considered as a CSTR, predicted by a model based on a zero-order reaction and diffusion inside the biofilm, reasonably agrees with experimental data.