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 (T-g) 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 T-g 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 T-g 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, 51 158-169, 2011 (C) 2010 Society of Plastics Engineers