The molar enthalpies of solution of a drug Albicar or 1,4-diethyl-3,6-dimethyltetrahydroimidazo[4,5-d]imidazole-2,5(1H, 3H)-dione in ordinary (H2O) and heavy (D2O) water at T = (278.15, 279.15, 288.15, 298.15, 308.15, and 313.15) K and ambient pressure were measured calorimetrically. The standard (at infinite dilution) molar enthalpies and heat capacities of the solute dissolution along with the (D2O-H2O) solvent isotope effects (IEs) on the specified quantities were computed. The enthalpic effects of Albicar dissolution in H2O and D2O experience a negative-to-positive sign inversion nearby T-inv = (306 and 307) K, respectively, whereas the corresponding IEs were found to be wholly negative (by sign) and decreasing in magnitude with increasing temperature. These facts indicate that the Albicar hydration, being dualistic (hydrophobic-hydrophilic) in nature, is enhanced in the D2O medium. The hydration behavior of the studied solute was discussed in comparison with those for Mebicar (1,3,4,6-tetramethyl-derivative) and Bicaret (1,3,4,6-tetraethyl-derivative) using the previously obtained data. (C) 2016 Elsevier Ltd. All rights reserved.