We report the thermal, mechanical, and diffusion properties of bisphenol E based polycyanurate nanocomposites with three forms of graphene derived from sequential processing of the same carbon nanostructure. Edge-functionalized graphene nanoplatelets (GNP) were converted to graphene oxide (GO), then heated to produce thermally reduced graphene oxide (TRGO). All three reinforcements were individually mixed with the dicyanate ester of bisphenol E (LECy) at low loading levels and cured to form polycyanurate nanocomposites. GNP, with very low oxygen functionality, was incompatible with the cyanate ester, while the highly oxidized GO formed well-dispersed (though not exfoliated) nanocomposites, with the TRGO forming a good dispersion on mixing but phase separating during cure. The addition of GO, and, to a lesser extent, TRGO, resulted in improved mechanical proper-ties, particularly fracture toughness, with the addition of TRGO having a modestly negative effect on the glass transition temperature. Surprisingly, neither GO nor TRGO addition was effective at slowing down the diffusion of water in the polycyanurate, with the addition of both resulting in increased equilibrium moisture uptake. It thus appears that the trade-off between dispersion and the required level of oxygen functionality acts in a manner to frustrate attempts at minimizing the permeation of water by addition of graphene-based reinforcements. (C) 2014 Wiley Periodicals, Inc.