Studies on the solubility of 1,3,5,7-tetraazatricyclo[3.3.1.13,7]decane in water do not agree about the dependence of the solubility of this compound with temperature. In the present work, transition points of the macrocyclic aminal 1,3,5,7-tetraazatricyclo[3.3.1.13,7]decane or hexamethylenetetramine (HMT) in saturated aqueous solution were determined using refractive index and viscosity measurements between 275.15 and 313.15 K. The behaviour of both properties as a function of temperature shows three different trends. The transition temperatures were determined from the intersection points in the graphs. In this way two transition points were obtained: the first is observed at 284.14 K and the second at 299.11 K. The results suggest that there are at least three solid forms in equilibrium with the solution in the temperature range considered.