The effect of moisture during cure on the mechanical and thermal performance was evaluated for a series of cyanate ester composite laminates. Data presented in our investigation show that the amount of carbamate formation is inversely proportional to the glass transition temperature (Tg) of the cured composite and is typically localized on the outer surface plies adjacent the moisture source. Even though the majority of the composite may remains largely unaffected, the Tg of the localized areas will show sharp decreases which correlate with strength as verified by flatwise tensile tests. Additionally, our test data show that performing just one Tg measurement after cure will not necessarily provide a valid assessment of the state of the composite since additional degradation may occur with subsequent thermal exposure. Accordingly, a second dynamic mechanical analysis (DMA) measurement should be performed after an extended, elevated temperature, post-treatment thereby allowing direct assessment of carbamate degradation. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers.