The lower critical solution temperature (LCST) of cyclotriphosphazenes and polyphosphazenes grafted with a hydrophilic poly(ethylene glycol) (PEG) and a hydrophobic amino acid ester (AAE) as side groups could be quantitatively correlated with the overall hydrophobicity given by linear combination of the logarithmic partition coefficients of the corresponding free PEG and AAE measured in the 1-octanol/water system. Thus the LCSTs of phosphazenes were found to be well described as a linear function of a new parameter P-t defined as linear combination of the logarithmic partition coefficients of the free side groups, K-peg and K-aac : P-t = xlog K-peg + (2 - x)log K-aae where x is the mole fraction of PEG. The LCST of cyclotriphosphazenes has shown to be more sensitive to change in hydrophobicity than polyphosphazenes, indicating that the nature of the molecular structure plays an important role in determining their LCST. The additivity of the logarithms of the two partition coefficients for the parameter may indicate that the contribution of each constituent is independent, providing a facile method for prediction of the LCST of thermosensitive polymers. (C) 2004 Elsevier Ltd. All rights reserved.