Starting with hexachlorocyclotriphosphazene (HCCP) and branched polyethylenimine (bPEI) crosslinked cyclomatrix polyphosphazene poly(HCCP-co-bPEI) microspheres are prepared by precipitation polymerization in acetonitrile at 50 degrees C under ultrasound irradiation. The influence of varying reagent ratios on the chemical composition, morphology, and physical and chemical properties of the microspheres is investigated. Furthermore, the microspheres are characterized by Fourier-transform infrared spectroscopy, dynamic light scattering (DLS), X-ray photoelectron spectroscopy, elemental analysis, as well as P-31 MAS NMR. FESEM and DLS analysis show microspheres of smooth and spherical shape which swell both in water and acetonitrile. The particle diameters in the dry state range from 0.4 to 0.9 m, but due to swelling a size increase between 45 and 140% is observed depending on the chemical composition and the solvent medium. The thermal stability of the microspheres ranges from 262 to 351 degrees C. The presence of reactive amine groups on the surface is proven by electrophoretic mobility measurements and chemical modification with two model compounds. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 527-536