N-Methylformamide (NMF)-based matrices for capillary electrophoretic separation of nucleic acids have been developed. The use of an organic solvent as liquid base for the separation matrices allowed a hydrophobic polymer, C16-derivatized 2-hydroxyethyl cellulose (HEC), to be employed as structural element in the sieving medium. With a matrix consisting of 5% w/v of this polymer dissolved in NMF containing 50 mM ammonium acetate, p(dA)12-18 and p(dA)40-60 oligonucleotides were baseline separated. The addition of ammonium acetate to the buffer and separation matrix resulted in enhanced separation efficiency. Furthermore, it was possible to tailor the sieving performance of the separation medium by the use of a binary mixture of C16-derivatized HEC and PVP. Differences in sieving behavior of the various matrices evaluated are discussed.