Three cyanate ester monomer or oligomer species: 2,2-bis(4-cyanatophenyl)propane 1, 1-1-bis(4-dicyanatophenyl)ethane 2, and the oligomeric phenolic cyanate 3, are blended in various ratios to form binary mixtures (18 in total), formulated with copper(II) acetylacetonate (200 ppm) in dodecylphenol (1% w/v active copper suspension) and cured (3 K min(-1) to 150 degrees C + 1 h; 3 K min(-1) to 200 degrees C + 3 h) followed by a post cure (3 K min(-1) to 260 degrees C + 1 h). Cured copolymers were exposed to environments of elevated relative humidity (75% RH) and parallel immersion testing in H2O, H2SO4 (10%) and NaOH (10%) at 25 degrees C for a period of up to 2 years and accelerated ageing in boiling water (14 days). Periodic measurements are made of moisture gain along with infrared spectra and compared with cured homopolymers. Changes in mass are recorded periodically throughout exposure, prior to destructive thermo-mechanical analyses. Dynamic mechanical thermal analysis data comparing neat and exposed blends demonstrate the detrimental effect of moisture ingress whilst data from thermogravimetric analysis demonstrate no change in degradation onset between neat and exposed materials. An optimised blend of 1:1 of monomer units 1 and 2 was found to absorb less moisture than blends of different stoichiometry or between other respective monomeric units, consequently limiting the deleterious effect of moisture ingress. (C) 2016 Elsevier B.V. All rights reserved.