The diffusion of silica particles with radii ranging from 12 to 510 nm in dilute solutions of carboxymethyl cellulose (M w = 180 to 1200 kg mol −1 ,c CMC = 5 to 1000 mg l −1 ) was investigated by means of dynamic light scattering at pH 5 in 0.01 mol l −1 NaCl. The viscosity of the polymer solution as experienced by the silica probes (the “microscopic” or effective viscosity, η eff ) differs from the viscosity as determined by capillary viscometry (η p ). For small particles η eff nearly equals the viscosity of the solvent (η 0 ). The effective viscosity increases with the size of the probe particles and the polymer concentration but remains less than η p . The effective viscosity is interpreted in terms of a model in which the particle is surrounded by a layer of polymer free solution (η = η 0 ). The thickness of the polymer-free layer is assumed to be equal to the thickness of the depletion layer (λ d ). Applying this model, a decrease in λ d as a function of CMC concentration is observed. At low concentration λ d equals the radius of gyration. The hydrodynamic layer thickness (δ h ) of cellulose derivatives (carboxymethyl cellulose and hydroxyethyl cellulose) adsorbed on inorganic oxide surfaces (α-Fe 2 O 3 and SiO 2 ) is also investigated by dynamic light scattering. Upon using η p maxima in δ h are found. However, these maxima are a consequence of an incorrect choice of the viscosity. When the viscosity is used as obtained from inert probe diffusion, no anomalies are observed.