The present invention is directed to a process for the isothermal nitration of aromatic organic compounds, wherein an aromatic organic compound is brought into contact and reacted with a mixture of sulfuric acid and nitric acid. The mixture of sulfuric acid and nitric acid, and the aromatic organic compound are brought into contact with each other through a mixing nozzle which comprises a substantially cylindrical tubular reactor and an annular gap which surrounds the tubular reactor. The aromatic organic compound flows through the tubular reactor, and the mixture of sulfuric acid and nitric acid flows through the annular gap, and they are mixed together upon emerging from the tubular reactor and the annular gap, wherein the ratio of the average rates of flow of the aromatic organic compound in the tubular reactor, and of the mixture of sulphuric acid and nitric acid in the annular gap ranges from 0.5:1 to 10:1.