Nitrification is a key process in nitrogen cycling. Several nitrification rate measure- ment techniques rely on the use of chemical inhibitors that effectively and specifically inhibit nitri- fying bacteria. The discovery of ammonia-oxidizing archaea has raised the question of whether these inhibitors are also effective for archaea. The efficiency of 2 commonly used chemicals — allylthiourea (ATU) and nitrapyrin (commercial name N-serve) — in inhibiting nitrification was tested in natural seawater samples containing archaea. In addition, the effect of these inhibitors was tested in a pure culture of Pseudomonas chlororaphis, a bacterium that is used in the denitri- fier method to reduce nitrate to nitrous oxide, whose isotopic composition can be analyzed with isotopic ratio mass spectrometers. This method is commonly used in stable isotope based nitrifica- tion rate measurements. ATU inhibited nitrification by 80% at a 100 μM concentration, whereas N-serve inhibited nitrification completely at a 150 μM concentration. Although the inhibition of nitrification by ATU was not complete for the natural seawater ammonia-oxidizing community, <100 μM concentration has been shown to be efficient. For N-serve, the previously found effective concentration (150 μM) for the natural ammonia-oxidizing community inhibited nitrification com- pletely. Both ATU and N-serve reduced nitrous oxide production in the denitrifier method, but the 15 N atom% values were not affected. The nitrification rates measured with inhibitors should be interpreted with caution because inhibitors can alter the functioning of bacterial methods used to analyze the nitrification rates.