The analysis of cured resin blends comprising cyanate ester and bismaleimide (BMI) to determine the chemical effects of long-term exposure to water is reported. The cured resin blends underwent accelerated water uptake by immersion in water at temperatures of up to 70 degrees C, for a period of 18 months. Blistering and severe micro-cracking occurred, leading to severe weakening of the worst affected blends. Photomicrographs confirmed that the worst affected resins, containing high BMI contents, displayed two-phase morphologies. Thermo gravimetric analysis and dynamic mechanical thermal analysis were performed on neat resin samples before and after immersion. Spectroscopy (near infrared Raman, diffuse reflectance and mid-infrared) was performed on powdered resin samples to determine the nature of the chemical changes occurring during the degradation. The elemental composition of the samples was also determined before and after conditioning.