Literature data on the isotopic composition of nitrous oxide indicate a general predominance of the α-15N-isotopomer and a parallel 18O-enrichment in N2O from nitrification and denitrification, respectively. As the kinetic isotope effects on any individual reactions of the two processes lead to depletions of the heavy isotopes of nitrogen and oxygen in the products, the observed enrichments could mainly be caused by enzymatic reduction of NO, provided it occurs via a symmetric intermediate like hyponitrite; infrared data are in favour of large differences between the binding constants of the isotopomers and isotopologues of this compound. As a matter of fact one of the mechanisms discussed for the nitric oxide reductase from certain microorganisms implies the parallel binding of two NO molecules and the formation of a symmetrical intermediate, while that of the enzyme from other microorganisms reduces NO in a sequential mechanism. In addition, isotope effects on the reduction of N2O to N2 must contribute to the observed isotope characteristics of N2O, especially in context with denitrification. Therefore, the known enzymatic reaction pathways suggest that the α-15N-isotopomer preference and the 18O-signature of the produced N2O is not essentially characteristic for its origin from nitrification or denitrification, respectively, but rather from the involved population of microorganisms and the type of their nitric oxide reductases. This has to be confirmed experimentally. Copyright © 2004 John Wiley & Sons, Ltd.