AbstractEvaluations of apparent slip and polymer adsorption are reported for laminar capillary flow of dilute aqueous solutions of the three homologues WSR 301, Coagulant and FRA of Polyox. Measurements were carried out using glass tubes coated with a silane compound (dimethyldiethoxysilane) as well as for the untreated glass tubes. The results indicate that flow enhancement is dominant at the very low polymer concentrations and flow retardation is dominant at the higher concentrations comprising the polymer concentration range investigated. A transition from a positive to a negative effective velocity at the wall was observed with increasing polymer concentration.A new analysis was applied to separate the contributions of polymer adsorption and slip in the evaluation of the effective velocity at the wall. Effective hydrodynamic thicknesses of the adsorbed polymer layers are presented as a function of the polymer molar mass and concentration and the wall shear stress. The thickness of the adsorbed layer at zero shear was also evaluated from the capillary flow data.