It has been thought that metastable peroxonitrite, ONOO–, does not interfere with the determination of nitrite, NO2–, because of evidence that it decays by isomerization to NO3–. In fact, the isomerization process is quantitative only under acidic conditions; partial decomposition to NO2– can take place in neutral solutions and quantitative decomposition to NO2– can occur in basic media with a metal catalyst. A method of analysis of these systems, based on these findings, is described. The details of the solution behaviour of ONOO– have not been recognized in the 50 years of studies of the photochemistry of nitrates where both NO2– and ONOO– are produced. As a result analyses were in error and changes in concentration of ONOO– with experimental variables have frequently appeared as changes in NO2– content, which were theoretically inexplicable.