A novel method of oxidation of poly(alkylene H-phosphonate)s is presented here. Trichloroisocyanuric acid is used for the first time as a chlorination agent of poly(alkylene H-phosphonate)s. Poly(oxyethylene chlorophosphate) was obtained in quantitative yield at room temperature in 1 h from poly(oxyethylene H-phosphonate) and trichloroisocyanuric acid. Poly(alkylene chlorophosphate)s have been used as an efficient polymer carriers of low molecular biologically active substances bearing hydroxyl or amino groups. Using different molar ratio between trichloroisocyanuric acid and poly(alkylene H-phosphonate) can control the degree. of oxidation, and novel co-polyphosphoesters with new properties can be obtained.