Conformational transitions of hydrogels of poly(methacrylic acid) induced by complexation with poly(ethylene glycol) were investigated. The degree of swelling of the interpolymer complex depends strongly on the pH of the solution and on the concentration of poly(ethylene glycol). It was shown that the addition of poly(ethylene glycol) results in sharp conformational transitions of neutral poly(methacrylic acid) gels in the collapsed state due to complex formation between the network and the linear polymer. The addition of the excess of poly(ethylene glycol) leads to reentrant swelling of the gel. A phase transition of the complex PMAA gel-PEG in the collapsed state determined by the concentration of low molecular weight acid in the solution was observed.