We investigate here solution properties of the common polysaccharide dextran (with various molecular weight fractions: T70, T110, T500, and T2000) in aqueous and in formamide solutions at 20 degrees C and 40 degrees C, as obtained from viscosity measurements. Dextran behaves as a random coil in water and formamide solutions on the basis of the Mark-Houwink-Sakurada constant being in the range 0.5< alpha < 0.8 and the molecular weight dependence of the hydrodynamic coil radius, Rcoil similar to MW1/2. Dextran segments interact more favorably with the formamide solvent molecules than with water, as attested by higher intrinsic viscosity, Rcoil, and hydrodynamic expansion factor alpha(eta) values in formamide compared to aqueous solutions. This can be attributed to formamide offering more readily available sites (C=O and NH3) to form hydrogen bonds with dextran. Apparently the stronger cohesive forces (reflected in higher surface tension s and solubility parameter delta values) between water molecules make it harder for dextran to break them, thus the dextran coils swell less in this medium. As the temperature increases from 20 degrees C to 40 degrees C, dextran is driven toward theta conditions in both water and formamide solutions. Upon heating, the polymer segments interact less with the solvent molecules and adopt a less expanded conformation in the solution; the solvent quality decreases, and the coils contract. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012