We report a study of the compatibility of a system involving a flexible polymer, hyaluronic acid (HA), and a semirigid polymer, (hydroxypropyl)cellulose (HPC). The phase diagram of ternary mixtures composed of HA, HPC, and H2O showed a homogeneous solution at an overall polymer concentration lower than C-p approximate to 15%, but when the concentration was increased, two demixing areas were evidenced : the first (range C-p 15-21%) corresponds to an equilibrium between two isotropic phases, while the second (over C-p = 21%) represents the coexistence of a HPC mesophase with an isotropic phase of both polymers. In contrast, optical and electron microscopy observations as well as X-ray, FT-Raman, DSC, and DMTA experiments carried out on blended films showed complete HPC/HA compatibility in all the compositions. DMTA experiments revealed a single glass transition temperature (T-g); its dependence as a function of composition was analyzed by Kwei’s equation. Different values were found for dry and 65% relative humidity (RH) conditioned samples, suggesting the important role of water as a plasticizer in HPC/HA blends.