Ultra-high viscosity alginates were extracted from the brown seaweeds Lessonia nigrescens (UIIVN, containing 61% mannuronate (M) and 2% guluronate (G)) and Lessonia trabeculata (UIIVT, containing 22% M and 78% G). The viscoelastic behavior of the aqueous solutions of these alginates was determined in shear flow in terms of the shear stress sigma(21), the first normal stress difference N-1, and the shear viscosity eta in isotonic NaCl solutions (0.154 mol/L) at T = 298 K in dependence of the shear rate. for solutions of varying concentrations and molar masses (3-10 x 10(5) g/mol, homologous series was prepared by ultrasonic degradation). Data obtained in small-amplitude oscillatory shear (SAOS) experiments obey the Cox-Merz rule. For comparison, a commercial alginate with intermediate chemical composition was additionally characterized. Particulate substances which are omnipresent in most alginates influenced the determination of the material functions at low shear rates. We have calculated structure-property relationships for the prediction of the viscosity yield, e.g., eta-M-w-c-(gamma) over dot for the Newtonian and non-Newtonian region. For the highest molar masses and concentrations, the elasticity yield in terms of N-1 could be determined. In addition, the extensional flow behavior of the alginates was measured using capillary breakup extensional rheometry. The results demonstrate that even samples with the same average molar mass but different molar mass distributions can be differentiated in contrast to shear flow or SAOS experiments.