Semidilute (above the overlap concentration, c*) aqueous solutions of ethyl(hydroxyethyl)cellulose (EHEC) as a function of concentration were investigated with the surface force apparatus (SFA), and by static light scattering (SLS) and osmotic pressure measurements. The anomalous excess in scattering intensity observed in SLS experiments at small angles indicates concentration inhomogeneities in the samples, probably as a result of association of the EHEC chains. The light scattering and osmotic pressure results show that EHEC in aqueous solution does not behave as expected for a polymer in a good solvent. The results from the SFA experiments show that EHEC adsorbs strongly on mica. These adsorbed layers give rise to a short-range steric repulsion when the two EHEC-coated mica surfaces are brought into close contact. The most remarkable result, however, is the very long-range attractive force observed at larger surface separations. This attractive force features oscillations with a periodicity that decreases when the polymer concentration increases. The periodicity corresponded to the mesh network size determined by SLS. To our knowledge, this type of long-range attraction has not previously been reported. The origin of the force could not be explained in terms of bridging or classical depletion effects. Rather, this long-range attraction might be induced by segregation of different polymer fractions, with different surface activities, present in the EHEC solutions.