The effect of PEGylation on the aqueous solution properties of a pH responsive pseudopeptide, poly(L-lysine iso-phthalamide), has been investigated using UV and fluorescence spectroscopy, H-1 NMR spectroscopy and dynamic light scattering. It was demonstrated that the level of PEGylation had a critical effect on the pH response of the parent polymer. When the degree of PEGylation was less than 23.4 wt% the modified polymer exhibited a transition from an expanded structure at high degrees of ionization to a compact hydrophobically stabilised structure at low degrees of ionization. The specific pH at which the conformational transition occurred was dependent on the degree of PEGylation. At levels of PEGylation in excess of 25.6 wt% the polymer no longer displayed this pH dependent conformation and existed in a micellar form (100-200 nm) over the whole range of ionization. Both linear and micellar forms of the pseudopeptide have applications in drug delivery. (c) 2004 Published by Elsevier Ltd.