Titration curves were obtained for poly(ethylenimine) (PEI), lauryl-containing PEI, and poly(allylamine) (PAA). Since each polymer contains more than 1000 amino groups, the (\mathrm{p} K_{\mathrm{a}}) values of individual amino groups are needed for rigorous analysis of the titration curves. Instead, the titration curves are empirically analyzed by defining the deprotonation quotient (Q) as the ratio of the fraction of unprotonated amino groups and the fraction of protonated ones and by assuming that (\log Q) is linearly related to (\mathrm{pH}). Very low basicity of the amines at acidic (\mathrm{pH}) 's is attributed to the electrostatic suppression of the protonation of amines in cationic microenvironments. Introduction of lauryl groups to PEI lowers basicity of the amines further, which agrees with unfavorable protonation of amines in hydrophobic domains. PAA, a linear polyamine, manifests basicity comparable to or smaller than those of PEI. This is taken to suggest that the hydrophilic portions and the hydrophobic portions of PAA form separate clusters. Implications of the results to various biomimetic functional molecules built on PEI or PAA derivatives are discussed. 1994 Academic Press. Inc.