Samples of H 2SO 4/SiO 2 loaded with HNO 3 and analysed by Raman spectroscopy exhibit the band of the NO 2 + ion, which is formed by the protonation–dehydration equilibrium of HNO 3 in strong acidic media. The effectiveness of NO 2 + as a nitrating species is tested in the nitration of aromatic compounds towards substrates with high acid requirements for the conversion reagents–products. The acidic properties of the acid systems used as catalysts are also described, and the thermodynamic parameters related to the acid–base proton transfer process are discussed.