Chemical modification of a chloromethylated polysulfone (CMPS) was performed by reacting the chlorometyl group with P-H bond of 9,10-dihydro-oxa-10-phosphophenanthrene-10-oxide (DOPO). A kinetic study of phosphorylation of CMPS with different chlorine content was reported. The obtained polymers bearing cyclic bulky groups containing phosphorus were characterized by analytical methods; Fourier transforms spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR). The thermal stability of phosphorus-modified polysulfone was evaluated by dynamic thermogravimetric analysis in nitrogen gas. The polymers were readily soluble in polar organic solvent, such as N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAc), and N,N-dimethylformamide (DMF). Differential scanning calorimeter (DSC) and dynamic mechanical analysis (DMA) proved an amorphous morphology of these polymers. POLYM. ENG. SCI., 50:48-56, 2010. (C) 2009 Society of Plastics Engineers