Two new thermally stable polyphosphonates were prepared from dichlorophenyl phosphine oxide and bisphenols with pendant heterocyclic ring by interfacial polycondensation using a phase transfer catalyst at 0 degrees C. They were characterized by IR, H-1 and P-31 n.m.r. spectroscopy. These polymers are amorphous in nature and readily soluble in chlorinated aliphatic hydrocarbons, such as CH2Cl2, CHCl3 etc., as well as polar aprotic solvents such as DMF, DMAC, NMP etc. Thermal and flammability studies were carried out by t.g.a., DTA and limiting oxygen index determination, respectively. Polymers are self extinguishing and begin to lose weight at around 350 degrees C.