Isotope composition of nitrite, nitrate, and water, as well as concentrations of nitrite, nitrate, and ammonium, were measured during the experiments with enrichment cultures of the anammox bacterium Kuenenia stuttgartiensis. Samples were collected sequentially during the course of the anammox activity until the complete consumption of nitrite. Isotope composition of nitrite and water was measured during the abiotic experiments to follow the abiotic oxygen isotope exchange between nitrite and water. The exchange was followed over the time period of 48 h.