The moisture absorption of cyanate ester modified epoxy resin matrices has been studied under constant hygrothermal conditions in an attempt to understand the so-called 'reverse thermal effect'. It was found that the resin exhibits the reverse thermal effect whereby the concentration of water absorbed is enhanced when the coupons are transferred from a high conditioning temperature (50 or 70 degreesC) to a lower one (20 degreesC). Free volume calculations using the model of Adamson [1] at different conditioning temperatures showed that the free volume of the resin decreases with increase in conditioning temperature. From the 'swelling' study, it can be argued that in the initial stage of the absorption process the water diffuses into the regions of volume equal to or greater than volume of water molecules which does not result in swelling (unbound water). In the later stage, the water molecules penetrate the regions with a volume less than that of the volume of water molecules (bound water), with molecular reorganization of the resin network, resulting in swelling.