During the degradation of the propellant stabilizer Centralite I the2-nitro-N-nitroso-N-ethylaniline 2N-NO-EA is formed showing two signals in the chromatographic analysis (HPLC) with a 2:1 ratio, while the 4-nitro isomer presents only one signal. This completely unexpected behaviour which interferes with the qualitative and quantitative determination was supposed to be due to the presence of two isomeric forms; however, the existence of two rotameric forms cannot be excluded. A detailed parameter study of the analytical conditions showed that at temperatures of 5 degrees C the isolation of the two forms should be possible. Preparative chromatography on a RP-18 column at 5 degrees C indeed gave fractions with differing percentages of both forms; at room temperature a re-equilibration to the starting ratio of 2:1 was observed. The same type of isomerism was found with three further N-alkyl derivatives of 2-nitro-N-nitrosoaniline bearing methyl-, propyl- or butyl substituents, respectively. It was confirmed by NMR spectroscopy that this phenomenon is caused by a partial double bond characteristic of the N-N bond in 2-nitro-N-nitroso-N-alkylanilines. The measured dependence of the percentage of both isomers on the size of the substituents makes clear that the isomer 'S' is predominating (60% up to 95%).