Four amphiphilic pyranines in which the acidic hydrogen of pyranine was replaced by an octyl, dodecyl, hexadecyl, and eicosyl group, POCn (n = 8, 12, 16, and 20), were prepared., and their spectroscopic properties and aggregation behaviors in water were investigated. The critical micelle concentration (cmc) of the amphiphilic pyranines was found to be relatively large even in POC20 having a long hydrophobic alkyl chain (ca. 3 x 10(-3) M). H-1 NMR studies revealed that POC16 and POC20 exist in the compact structure with the pyranine nucleus wrapped with a long methylene chain in water. As in the case of parent pyranine, the addition of methylviologen (MV2+) to an aqueous solution of POCn resulted in the absorption spectral change and efficient quenching of POCn fluorescence. In the case of POC8 and POC12, these spectral changes induced by the MV2+ addition were thoroughly explained in terms of the formation of the electrostatic complex POCn/MV2+ with a complexation constant of similar to 3 x 10(4) M-1. On the other hand, an unexpectedly large dependence of the absorption spectral change, as well as fluorescence quenching, on the total concentration of MV2+ was observed in POC16 and POC20. The Stern-Volmer plot for quenching of the fluorescence of POC16 and POC20 gave a curve deviating largely upward from a straight line. The plot was successfully analyzed by the equation induced by assuming the aggregate formation of the complex POCn/MV2+, which revealed the considerably small cmc values of the complexes, 3.6 x 10(-7) and 3.6 x 10(-8) M for POC16/MV2+ and POC20/MV2+, respectively. Experimental evidence in support of the aggregate formation was obtained by H-1 NMR and dynamic light scattering studies.