<p>The structure of interpolymer complex formed by poly(2-ethyl-2-oxazoline) and poly(methacrylic acid) was changed by controlling the hydrogen bonds between the polymer chains by changing the solvent pH or adding urea. The viscosity of solution containing the interpolymer complex drastically increased in solvent at pH~12, then decreased at pH~13. The infrared spectra of the interpolymer complexes formed in solvents at different pH indicated that the formation of hydrogen bonds among the polymer chains decreased with an increase of pH. These results were explained by a model in which the dimension of the interpolymer complex increased at pH~12, and the interpolymer complex dissociated into free polymer chains at pH~13. The hydrogen bonds of the interpolymer complex were also suppressed by adding urea to solvents. The viscosity of the solution containing the polymer-complex increased drastically when the urea concentration was over 6 M. The increase induced by the addition of urea can also be explained by the increase in the dimension of the interpolymer complex.</p>