A 15-day experiment was performed on male Wistar rats allocated into three blocks diversified by a daily thiamine dose: 0, 20, 40 μg/day/rat. In each block the rats were divided into five groups depending on the type of diet: control group (FF) – fructan-free diet; experimental groups (I-5, I-10, OF-5, OF-10) – diets containing: 5%, 10% of inulin, as well as 5%, 10% of oligofructose respectively. Thiamine-free experimental diets were prepared according to AIN-93M recommendations, where inulin and oligofructose were added instead of wheat starch. The thiamine was administered per os as a water solution. Compared to the initial value, caecal pH decreased in all groups of rats. The daily thiamine dose as well as the kind of fructan influenced caecal pH. There could be observed the synergistic action of inulin and thiamine in decreasing pH . Inulin and oligofructose, due to prebiotic properties, can cause fluctuations in the caecum pH but the direction of changes is closely dependent on the presence of dietary thiamine. The most suitable pH values for endogenous thiamine uptake are reported during dietary deficit of this vitamin, independently on the type and dose of fructan.