The four previous isopiestic investigations of Cu(NO3)(2)(aq) at the temperature 298.15 K exhibit a remarkable lack of agreement. Consequently, isopiestic vapor-pressure measurements were performed for Cu(NO3)(2)(aq) solutions at T = 298.15 K both at the Texas Christian University and the Lawrence Livermore National Laboratory. These measurements extend from molality m = (0.0638 to 6.9235) mol.kg(-1). Under the conditions typically used for isopiestic experiments, it is found that solutions of Cu(NO3)(2)(aq) at higher molalities lose HNO3 into the vapor phase, resulting in a gradual increase in the extent of hydrolysis of their solutions as the samples are re-equilibrated. This produces a progressive downward drift in the apparent osmotic coefficients. Consequently, most previous higher-molality isopiestic results for this system are believed to be unreliable. Experiments were performed to quantify and to minimize the effect of this hydrolysis on the resulting osmotic coefficients. These experiments indicate that the error in the osmotic coefficient from this HNO3 loss is insignificant at lower molalities of Cu(NO3)(2)(aq); at m = 4 mol.kg(-1) it is less than or equal to 0.2 per cent, but above m approximate to 5 mol.kg(-1) it can become quite significant. Recommended values of the osmotic coefficients, water activities, and mean activity coefficients of Cu(NO3)(2)(aq) are presented up to m = 3.00 mol.kg(-1), the maximum molality where we consider them to be completely accurate, and, with slightly lower accuracy, up to m = 6.9235 mol.kg(-1). Pitzer＇s equation was found to represent reliably the experimental osmotic coefficients provided the third virial coefficient was ionic-strength dependent.