AbstractThe weight‐average molecular weight of a fraction of magnesium alginate was found by light‐scattering to be identical within experimental error to that of the corresponding fraction of sodium alginate. The difference in the charge density of sodium and magnesium alginate had no detectable effect on their measured radii of gyration at ionic strength 0.1, but had a great effect on the second virial coefficient of the solutions.The Orfino‐Flory theory was used to calculate expansion factors, α, from the second virial coefficients, assuming that these were composed of two terms, one due to polymer‐solvent interaction and one due to electrostatic interactions. A comparison with α‐values obtained from viscosity data at different ionic strengths suggested that alginate is soluble in water because of electrostatic stabilisation and not because of a favourable polymer‐wate