The water solubility of 1,3,5,7-tetra azatricyclo[3.3.1.1(3,7)]decane, hexamethylenetetramine (HMT) or urotropine was determined at temperatures between 275.15 K and 313.15 K. Van't Hoff graphs of In m(sat) versus 1/T gave three different straight lines, suggesting the existence of three different forms of the substance in the temperature region studied. Previous work reported both increasing and decreasing solubility with temperature. This work found the solubility does not have a simple behavior. (c) 2006 Elsevier B.V. All rights reserved.