The purity of alkylnitrile compounds has been investigated using gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS) and high performance liquid chromatography (HPLC). The investigation was carried out to supplement earlier reported work using the alkylnitrile compounds, hexanenitrile and valeronitrile, as mobile phase additives in HPLC. GC impurity profiling and GC-MS experiments showed the presence of low level nitrogenous impurities in the alkylnitrile compounds. Analytical interrogation and chemical isolation were employed to identify the impurities as alkylamines. Tri-n-butylamine (TBA) and di-n-pentylamine (DPA) impurities were found in batches of hexanenitrile and valeronitrile respectively and it was found that it was the amine impurities, at very low levels, which caused the observed chromatographic improvements for basic solutes when using the alkylnitrile compounds as mobile phase additives in reversed-phase HPLC. The amines themselves were subsequently used at extremely low levels (ppm) in the mobile phase and were found to be very effective in their action.