Sedimentation, diffusion, and viscosity measurements were made on five unfractionated samples of sodium alginate ranging in intrinsic viscosity from 3.1 to 17.5. Diffusion coefficients were subject to large errors and are believed to be overestimated.Though the molecular weights obtained from sedimentation–diffusion (Svedberg equation) and sedimentation – intrinsic viscosity (Perrin–Simha equations) showed good agreement and yielded values of 3 to 21 × 104, higher values (4.6 to 37 × 104) from sedimentation–viscosity (Mandelkern–Flory equation) appear to be the better estimates. A linear relation between intrinsic viscosity and molecular weight was found with a slope (Mandelkern–Flory equation values) equivalent to Km = 13.9 × 10−3. The results indicate that sodium alginate has a relatively high extension ratio.