Affinities of polypyrrole for nitrite and nitrate ions can be exploited for use in ion sensing. Using modern microsensor fabrication technique this has led to the development of a highly stable, sensitive and fast-response sensor for nitrate and nitrite ions. Despite its poor differentiation between these two ions and despite cationic interferences from heavy metals present in high concentrations, it promises reliable monitoring of nitrate or nitrite ion in a well-defined environment.