4-Nitroaniline forms molecular adducts with 4-amino benzoic acid. It reacts with nitrite ion in hydrochloric acid medium to form 4-nitrophenyldiazonium chloride, which couples with naphth-1-ol in alkaline medium to give a purple azo dye. Photocatalytic degradation of 4-nitroaniline in the presence of TiO2 suspensions in a batch and continuous annular reactor has been studied.