One of the important factors, which determine the concentration of moisture that a polymer will absorb, is the effect of non-random mixing whereby water-clustering is said to occur. The Flory-Huggins theory cannot predict the isotherms observed for highly polar polymers since the theory assumes complete random mixing. The complementary model of Brown [Brown GL. In: Rowland SP, editor. Water in polymers. Washington, DC: American Chemical Society; 1980. p. 441. [1]] considers the moisture distribution in polar polymers, consists of components associated with random mixing (Flory-Huggins theory) and clustering of the water molecules. Moisture sorption isotherms of cyanate ester/epoxy blends at different relative humidities have been analysed by the clustering theory of Brown [Brown GL. In: Rowland SP, editor. Water in polymers. Washington, DC: American Chemical Society; 1980. p. 441. [1]] and the dual mode sorption theory of Zimm [Zimm BH. J Chem Phys 1953;21:934. [2]]. Clustering was found to occur in cyanate ester/epoxy blends conditioned at relative humidities of 40% and higher. The Cluster size indicates that the majority of the absorbed water in a cured cyanate ester/epoxy blend was not clustered but present in monomeric form through hydrogen bonding to polar sites. It is postulated that the phenomenon of thermal spiking enhanced moisture absorption can be explained by an equilibrium between cluster formation and declustering associated with network relaxation at the spike-temperature. (c) 2005 Elsevier Ltd. All rights reserved.