Isopiestic vapor-pressure measurements were made for Lu-2(SO4)(3)(aq) at molalities from (0.35379 to 0.66478) mol . kg-(1) and for H2SO4(aq) at molalities from (0.27470 to 0.70512) mol . kg(-1) at the temperature T = 298.15 K using KCl(aq) as the isopiestic reference standard; results for Lu-2(SO4)(3)(aq) extend to slightly above saturation. These measurements encompass the water activity ranges a(w) of 0.9902 less than or equal to a(w) less than or equal to 0.9783 and 0.9902 less than or equal to a(w) less than or equal to 0.9741, respectively. Molality analyses used for previous solubility, density (J. Solution Chem. 1988, 17, 499-517), and isopiestic measurements (J. Solution Chem. 1990, 19, 525-541) for Lu-2(SO4)(3)(aq) from this laboratory were found to be slightly high due to absorption of water vapor during the analyses, as was that of the NaCl(aq) isopiestic reference standard from incomplete drying during the dehydration analysis. Those results were corrected and combined with the present isopiestic measurements to yield recommended values of osmotic coefficients, water activities, and mean activity coefficients of Lu-2(SO4)(3)(aq), and the solubility and thermodynamic solubility product of Lu-2(SO4)(3) . 8H(2)O(cr), all at the temperature T = 298.15 K. A detailed study was made of the extent of the retention of water by NaCl(s) during drying and of absorption of water vapor by Lu-2(SO4)(3)(s) and other anhydrous rare earth sulfates as they are weighed in air. Methods were developed for accurately correcting each total mass of rare earth sulfate for absorbed water. In addition, isopiestic measurements were performed for H2SO4(aq) in the molality region where previous isopiestic results were found to be somewhat discordant in the critical review by Clegg et al. (J. Chem. Sec. Faraday Trans. 1994, 90, 1875-1894).