Terpolymer resins (BPhThF) were synthesized by the condensation of 2,2'-dihydroxybiphenyl [BPh] and thiourea [Th] with formaldehyde IF] in the presence of acid catalyst and using varied molar ratios of reacting monomers. Terpolymer resin compositions have been determined on the basis of their elemental analysis, and the number average molecular weights of these resins were determined by conductometric titration in nonaqueous medium. Viscometric measurements in dimethyl formamide (DMF) have been carried out with a view to ascertain the characteristic functions and constants. UV-vis, FTIR, and NMR spectra were studied to elucidate the structure. (C) 2010 Wiley Periodicals, Inc. J Appl Polyni Sci 118: 1245-1251, 2010