The phase behavior and the rheological properties of aqueous solutions containing mixtures of multiblock hydrophobically modified polyacrylamides (HMPAM) have been investigated, Mixtures of a HMPAM with a homopolyacrylamide (PAM) phase separate above a given overall polymer concentration and a given PAM content. These results are in qualitative agreement with the predictions of a model based on the entropy changes due to the associations between the hydrophobic units [Annable, T.; Ettelaie, R. Macromolecules 1994, 27, 5616]. The viscoelastic behavior of semidilute solutions of various series of copolymer mixtures with variable molecular weights (M-w approximate to 10(5) to 2 x 10(6)), hydrophobe contents ([H] = 0.5-2 mol %), and hydrophobic block lengths (N-H approximate to 2-7 units per block) has been studied using steady-flow and oscillatory experiments. The linear viscoelasticity can be described by a slow relaxation process and other faster complex relaxation processes. When both HMPAM are mutually entangled, the slow relaxation is in agreement with the expectation from a hindered reptation model with in particular a plateau modulus that only depends on polymer concentration. When the two HMPAM are unentangled, the behavior is more complex, the properties being strongly dependent on the hydrophobe characteristics of the two copolymers. In particular, when there is a strong mismatch between the hydrophobic parameters, the viscosity of the system is depressed likely due to a local segregation between the two copolymers.