Dynamic mechanical properties of aqueous solutions of hydroxypropyl methyl cellulose (HPMC) were investigated using oscillatory shear measurements. The structure was investigated with light scattering. A systematic investigation of the frequency dependence of the shear moduli showed that HPMC forms two distinct interpenetrating networks. A transient network is formed above about 0.3 wt % by reversible cross-linking of the chains. The elastic modulus of this network is independent of the temperature, but increases linearly with the concentration. An independent permanent network is formed involving a small fraction of the polymers and has an elastic modulus that increases with increasing temperature. Its elastic modulus is history dependent and evolves slowly with time. The transient network collapses at a critical temperature where micro phase separation occurs, but the permanent network is not influenced by this phenomenon. Light scattering showed that the pore size of the transient network is less than 40 nm, while probe diffusion measurements showed that the pore size of the permanent network is larger than 1 mu m.