Densities of (water + hexamethylphosphoric triamide) in the entire mole-fraction composition at five temperatures, from (288.15 to 308.15) K. and atmospheric pressure were measured by using a magnetic float densimeter with an error of +/-1.1 . 10(-5) g . cm(-3). Excess molar volumes of the mixtures and apparent molar volumes of the components (down to their infinite dilution) were calculated. The volumetric effects of mixing being very large in magnitude present negative deviations from ideality and become decreasingly negative with increasing temperature. The apparent molar volume of organic co-solvent displays a clearly pronounced minimum in the water-rich region at all the temperatures studied. It has been shown that there is a thermodynamically substantiated interrelation between volume and enthalpy (heat capacity) properties of the mixtures considered. (C) 2010 Elsevier Ltd. All rights reserved.