Polycyanurates were prepared by interfacial polycondensation of 2-methoxy-4,6-dichloro-s-triazine (MDT) jointly with bisphenol-A and bisphenol-C using a chloroform-water system and different molar proportions of the reactants. The copolymer compositions of the tercopolymer samples were estimated by analysis of NMR spectral data. One tercopolymer sample was fractionated by fractional precipitation. The fractions were characterized by measurements based on viscometry osmometry and gel permeation chromatography. The intrinsic viscosity data were analyzed in terms of the Mark-Houwink-Kuhn-Sakurada (MHKS) relation. The unperturbed dimensions and polymer-solvent interaction parameters were estimated by the application of the current theories of excluded volume effect.