The polycyanurate network matrix derived from the thermal, dibutyl tin dilaurate catalyzed polymerization of bisphenol A dicyanate was modified in their glass-laminate composites with different linear polymeric additives bearing pendant phenol, cyanate, and epoxy functions. The mechanical properties and fracture energy for delamination of the glass-laminate composites were estimated as functions of the backbone structure and concentration of the various additives. The effect of altering the nature or concentration of the functional group for a given backbone structure of the additive was examined in some cases. Except for the epoxy functional acrylic polymer, all other systems adversely affected the fracture energy for delamination of the composites due to either plasticization or embrittlement of the matrix. With the exception of the styrene-hydroxyphenyl maleimide (SPM) copolymer, the other modifiers impaired the mechanical properties and adversely affected the thermomechanical profile of the composites. In the cases of the phenol functional acrylic polymer and its cyanate derivative, plasticization of the matrix by the partly phase-separated additive, which eased the fiber debonding, appears to be responsible for the impairment of the mechanical properties. The high glass transition temperature SPM copolymer enhanced the resin-reinforcement interaction through dipolar interactions induced by the hydroxyl groups, which resulted in amelioration of the mechanical properties. However, its possible coreaction and formation of a brittle, homogeneous phase with the polycyanurate was conducive for poor damage tolerance. The SEM analysis of the fractured composites showed that in the elastomers fiber debonding is the major cause for delamination. Although the presence of SPM led to a stronger interphase, failure occurred either in the brittle matrix or through fiber breakage.