Densities of pure hexamethyleneimine and of its aqueous Solutions have been measured at atmospheric pressure, using an Anton Paar digital vibrating tube densimeter, from 273.16 to 363.15 K and from 283.15 to 353.15 K, respectively. Acceptable representations of experimental data are found using a general correlation from Dauber et al. for pure hexamethyleneimine and using a general correlation from Bettin and Spieweck for pure water: thus indicating consistency of newly measured data. In this paper, we show the Redlich-Kister equation leads to incorrect data treatment particularly at low concentrations for systems presenting hydrophobic interactions. Thermal expansion coefficients (alpha*) for the pure hexamethyleneimine (HMI) and excess thermal expansion coefficient (alpha(E)) of the binary mixture: HMI + water at 283.15 and 298.15 K over the whole mole fraction range of water are presented and discussed in terms of the Structural changes in the mixtures. (C) 2009 Elsevier B.V. All rights reserved.