Self-association properties of poly(N-methacryloylphenylalanine) and poly(N-methacryloyltryptophan) (pMPhe and pMTrp, respectively) were investigated by several characterization techniques, including steady-state fluorescence and NMR. These characterization data revealed similarities and distinctions of their self-association properties. The pH dependencies of association properties of pMPhe and pMTrp are practically the same. Apparent pK(a) values for pMPhe and pMTrp were determined to be 5.7 and 5.8, respectively, by potentiometric titration. Steady-state fluorescence measurements at varying pH using pyrene as fluorescence probe indicated that, at pH approximate to 5 (< apparent pH), hydrophobic microdomains were formed, while, at pH approximate to 7 and 9 (> apparent pK(a)), hydrophobic microdomains were not formed significantly. H-1 NMR spectra for both the polymers measured in D2O exhibited that a significant fraction of aromatic rings in amino acid residues were located close to the polymer main chain, and that, at pH approximate to 5, the mobility of the polymer main chain and the aromatic ring was extremely restricted. The polymer concentration (C,,) dependencies of association properties of pMPhe and pMTrp at pHapproximate to5 are distinct. Steady-state fluorescence data at varying C-p indicated that pMPhe was more hydrophobic microscopically than pMTrp. Dynamic light scattering data indicated that pMTrp had a stronger tendency for interpolymer association than pMPhe did at. pH approximate to 5. It is concluded that the distinction in CP dependency of the self-association properties of pMPhe and pMTrp is due to the differences in the bulkiness and the hydrophobicity of the substituents of amino acid residues. (C) 2004 Elsevier Ltd. All rights reserved.