The use of poly(2-ethyl-2-oxazoline) with a number-average molecular weight (of polymer) of 500 000 g/mol in a sieving matrix for SDS CE size separation of proteins is, investigated using polymer solutions with concentrations between 6 and 12%w/v Optimal separation efficiency is obtained using 10%w/v, with an average separation efficiency of 150 000 (12 000) theoretical plates observed for myoglobin (weight-average molecular weight = 18 kDa) and 8 800 000 (2 400 000) theoretical plates for carbonic anhydrase (weight-average molecular weight = 30 kDa) for 14 capillaries with an 80 cm effective length Some adsorbed polymers were found to be effective in reducing EOF when applied to a clean capillary, however, a covalently attached polyacrylamide coating was found to be much less troublesome after initial wall treatment. Two different buffer systems, (i) 25 mM HEPES and (ii) 20 mM Tris with 40 or 60 mM tricene, were tried with 0.05 or 0.10%w/v SDS added. Tris-tricene gave generally better current stability than HEPES but with no observed improvement in separation efficiency. This polymer has all the desired characteristics for an SDS-CE sieving matrix using LIF detection, including hydrolytic stability, optical clarity, low viscosity, acceptable hydrophilicity, and commercial availability.