Capillary electrophoresis of λ double-stranded (ds) DNA (48.5 kbp) in dilute hydroxyethylcellulose (HEC) solutions shows band spreading that cannot be explained by diffusion alone. Dispersion and asymmetry factors of λ ds-DNA bands were measured as functions of capillary length, HEC concentration and field strength. Band spreading and asymmetry can be explained by a recently developed model in which the dominant contribution is assumed to be dispersion in DNA–HEC disentanglement times. Bandwidth reduction using square-wave field inversion was also investigated. It is proposed that correlation of DNA motion is the source of band narrowing during field inversion.