The osmotic coefficients of 1,3,5,7-tetrazatricyclo (3.3.1.13,7) decane (HMT) and 1,3,6,8-tetrazatricyclo (4.4.1.13,8) dodecane (TATD) in aqueous solutions were measured at 288.15,293.15,298.15 and 303.15 K. The technique used was the isopiestic. The data were used to calculate the activity coefficients of the two solutes at the four temperatures. The few data points found for HMT in literature at 298.15 K agree well with the new data. The reinforcement of water structure by the two aminals is discussed. (C) 2010 Elsevier B.V. All rights reserved.