The interaction between polyacrylic acid and two water-soluble cellulose ethers (methylcellulose and hydroxyethylcellulose) was studied. Viscometry, velocity sedimentation, small-angle neutron scattering, and potentiometric titration methods show the formation of stable water-soluble interpolymer complexes due to hydrogen bonding between nondissociated groups of polyacrylic acid and proton-acceptor groups of the cellulose ether. Both complexes exhibit polyelectrolyte properties and keep the conformation of the semirigid chain component. The hydrophobic groups in the methylcellulose macromolecule are responsible for the absence of complexes at temperatures greater than 60 degrees C.