H-1 NMR spectroscopy was used to investigate the location of two nitroso compounds, N-methyl-N-nitroso-p-toluenesulfonamide and alpha-nitroso-beta-naphthol (the latter predominantly in quinonemono-oxime form), and the penetration of water, in ionic micelles. The head group and chain length of the surfactant did not influence the preferential location of the nitroso compounds, the aromatic rings of which were centered in the neighborhood of the beta or gamma carbons of the hydrocarbon chain. Possible extrapolation of results to biological structures is discussed in the context of the biological activity of carcinogenic nitroso compounds. The study of hydration showed that in alkyltrimethyl-ammonium micelles the surfactant head groups lay in a highly polar region. Hydration was considerable at the alpha carbon but very minor in more internal regions, except for a slight increase at the chain terminus that is attributed to chain looping. In N-cetylpyridinium chloride micelles, the head group was folded back on the chain to avoid immersion of the aromatic ring in the aqueous extramicellar phase.