The density of tetrapropylammonium bromide aqueous solutions has been measured with six-figure precision at T/K - 273.15 = (5, 10, 15, 20, 25, 30, and 35) and molality from 0.05 to 8 (at 18 closely spaced intervals) using a vibrating-tube densitometer. The apparent molar volume, as well as both the solute and solvent partial molar volumes and their derivatives with respect to temperature, were calculated and compared with those of the tetra-alkyl ammonium (R4N+, R: from methyl to butyl) halide solution series to detect structural transformations due to hydration and hydration interactions. Except for Me4N-Cl, the results indicate progressive solvent structure enhancement in the R4N+ series and significantly different solute-solvent ratios for the "ideal" spatial ("interstitial") and energetic co-sphere arrangements. (C) 2010 Elsevier Ltd. All rights reserved.