The postprandial glycaemic response following a meal is reduced with the addition of soluble dietary fibre. The reductions in the glycaemia are thought to be due largely to increased viscosity of the gastrointestinal (GI) contents retarding digestion and absorption. The aims of the present study were to determine the effect that the GI tract has on the viscosity of meals containing different soluble fibres and to determine whether the glycaemic response of a meal (containing the soluble fibre) was predicted by the viscosity of the digesta in the small intestine. High carbohydrate diets containing 70 g soluble fibre (guar gum, xanthan gum or methylcellulose)/kg or 70 g insoluble fibre (wheat bran)/kg were diluted in water to a final fibre concentration of 18 g/kg. Following dilution the wheat bran diet had no measurable viscosity, while the viscosities of the soluble fibre diets were elevated. When the diets were fed to male Sprague–Dawley rats for 2 weeks the viscosities of the stomach and small intestinal digesta were not predicted by the viscosity of the diets measured before ingestion The action of the GI tract on the viscosity of the soluble fibres was investigated in vitro by dilution of the diets with acidic and neutralizing solutions, mimicking gastric and duodenal secretions. Dilution of diets with either acidic and neutralizing solutions or saline control significantly lowered the viscosity of all diets, while alterations in the pH of the diets had little impact on the resultant viscosity. When fasted rats were orally administered with the differing diets (0.25 g carbohydrate/kg body weight), the postprandial glucose response was reduced following the soluble-fibre-containing meals when compared with the wheat bran-supplemented meal, although the reduction in glycaemia only reached statistical significance with xanthan supplementation. These results indicate that there are large changes in the viscosity of a meal containing soluble fibre following ingestion, and that dilution of the diet by GI secretions is important in determining the resultant viscosity in the small intestine. Furthermore, the large differences in viscosity of the GI contents following consumption of the diets containing the soluble fibres were not predictive of the postprandial glycaemic response.