A new isopiestic apparatus has been designed and constructed following several criteria. It consists mainly of several small sample cups for holding small quantities of reference standard solutions, and a big sample cup for a bigger quantity of a test solution. Using this apparatus, experiments on NaOH solutions have been performed. The experimental procedure, the consistency among the samples in equilibrium, the equilibration process, and the determined osmotic coefficients of NaOH solutions are discussed. The apparatus is found to ensure a consistent temperature among the samples in equilibrium, meeting the experimental requirements for samples of molalities less than 0.05 mol(.)kg(-1). Inside the apparatus, the temperature can reach the desired uniform temperature within less than 0.5 d. In the experiments, the equilibration process is essentially determined by changes in the reference standard solutions in the small cups. Thus the apparatus is not only reliable and stable, but is also suitable for experiments on solutions of viscous, complex and unstable solutes. The equilibration time of the experiments is fast, which is practical for samples of molalities less than 0.05 mol(.)kg(-1). Moreover, with the new apparatus it is easy to determine the end point of the equilibration. (C) 2003 Elsevier Ltd. All rights reserved.