A series of hyperbranched poly(3-methyl-3-hydroxymethyloxetane)s with different degree of branching were synthesized using BF3 center dot OEt2 as initiator and coated on the inner surface of the fused-silica capillaries. In the pH range of 3-9, the coated capillaries reduced electro-osmotic flow by about flow times lower than the bare fused-silica capillary. The coated capillaries also displayed good resistance to adsorption of cationic proteins, providing clean separations of a mixture of Lysozyme, Cytochrome c, and Ribonuclease A around pH 3-6 in phosphate buffer. The separation efficiency in terms of peak shape was excellent compared with bare fuse-silica capillary. The separation efficiency of hyperbranched poly(3-methyl-3-hydroxymethyloxetane) with degree of branching of 0.43-coated capillary column for Lysozyme reached 10(6) plates/m with a resolution of 7.1, and the coated capillary column had good migration time reproducibility. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 2473-2479, 2010