Two poly(ether urethane urea)s were synthesized, one based on poly(propylene glycol) and another one on poly(tetramethylene glycol). Hydrogenated MDI was used as the diisocyanate and propylenediamine as the chain extender. The diisocyanate : polyol : diamine molar ratio was 2 : 1 : 1 for both copolymers. Data from stress-relaxation tests were adjusted to a power law and to the Kohlraush-William-Watts equation. Phase separation and viscoelastic behavior were correlated through the calculation of the time-relaxation spectrum, the steady-state tensile compliance, and the tensile viscosity. The results indicated that the material based on poly(tetramethylene glycol) was the more effectively phase-segregated block copolymer.