The phase equilibrium of polymer-microemulsion mixtures has been studied. Two polymers with the same cellulose backbone have been examined in comparison : hydrophobically modified ethyl hydroxyethyl cellulose (HM-EHEC), containing ca. 6.5 hydrophobic "stickers" per molecule versus the unmodified precursor (EHEC). It has been shown that (i) the modified polymer is soluble in the bicontinuous microemulsion phase, whereas the unmodified polymer is not, (ii) HM-EHEC, being dissolved in the bicontinuous phase increases its ultimate swelling in water and, to a less degree, in oil, and (iii) these effects of HM-EHEC on the phase equilibrium disappear at high polymer concentrations. The influence of HM-EHEC on the microemulsion phase equilibrium is discussed in terms of the polymer effects on the surfactant monolayer elasticity.