The effect of calcium ions on the rheology of xanthan has been studied by low-amplitude oscillatory measurements. Solutions of Na + xanthan were prepared at a fixed concentration of 5g/l (∼7.5mN w.r.t. COO − ), with incorporation of 10mN NaCl to ensure adoption of the ordered conformation, and with CaCl 2 concentrations varied between 0 and 100mN. Measurements were made at 5°C after cooling (at 1°C/min) from the disordered state at 90°C. G′, G″ and the slope of logη ∗ vs. logω rose steeply as calcium ion concentration was increased to 7.5mN (∼100% stoichiometric equivalence), but then dropped sharply between 7.5 and 15 mN, before continuing to increase monotonically at higher concentrations of Ca 2+ ; converse behaviour was observed for tanδ. Thus the solutions pass through states of maximum and minimum gel-like character at ∼100% and ∼200% stoichiometric equivalence of Ca 2+ (i.e. at Ca 2+ :COO − ratios of ∼1:2 and ∼1:1, respectively). Closely similar results were obtained for Me 4 N + xanthan in the presence of 10mN Me 4 NCl and 4M urea. The anomalous reduction in gel-like character at Ca 2+ concentration between stoichiometric and twice stoichiometric is tentatively ascribed to partial replacement of intermolecular site-binding of calcium ions by binding to individual carboxyl groups, to maximise the degree of complexation.