AbstractDenitrification is studied in sediments from two areas of the River Butrón which represent different situations with respect to their degree of pollution. Denitrification is analysed by means of the following parameters: maximum denitrification rate (Vmax), Michaelis constant (Km), denitrification rate (v), denitrification constant (kd) and number of denitrifiers. In all the samples analysed denitrification follows a kinetic type Michaelis‐Menten with respect to nitrate.Vmax,Kmand number of denitrifiers are clearly superior in the polluted area when compared to the cleaner area, indicating an advantage given that denitrification suggests an alternative pathway for breaking down organic matter with low levels of dissolved oxygen. By comparison betweenKmand natural nitrate concentration values we find thatvfollows a first order kinetic depending directly on the nitrate concentration.valso presents higher values in the area of the river where the entry of sewage is taking place than in the cleaner area. However, this difference does not have a kinetic origin but rather is given by different nitrate concentrations in their sediments. This is due to the fact that the two areas present similar efficiencies in the elimination of nitrate, which is deduced from the similar values of