Bioactivity of polyphenolic compounds after intake depends mainly on transformations in the gastrointestinal tract and the structure of the metabolites produced. The objective of the research study was to determine the transformations of some selected polyphenolic compounds occurring during in vitro digestion thereof. For the experiment, solutions of ferulic and p-coumaric acids, (+)catechin, quercetin, hesperidin and hesperetin were used. They underwent a simulated digestive process, and, at some selected stages, the antioxidant activity (using a spectrophotometric method with ABTS^•+ radical cation) was determined and the contents were assayed of the compounds (with the use of a HPLC high-performance liquid chromatography). Of the analyzed compounds, quercetin was characterized by the highest antioxidant activity (234 g Trolox/g). Hesperetin (aglycone) showed a higher antiradical activity than hesperidin, its rutinoside. The digestion of the majority of polyphenols analyzed started in the stomach and duodenum, and the derivatives produced had a high antioxidant potential. Phenolic acids were most digestion prone in this part of the gastrointestinal tract. Also, it was found that the glycosides (hesperidin) were significantly decomposed by the gastric acid and duodenal enzymes. The polyphenols tested were characterized by a varying degree of migration through the dialysis membranes. The phenolic acids passed more easily through the membranes than the flavonoids; this fact could be related to a smaller size of the acid molecules. The intestinal bacteria used in the experiment impacted all the polyphenols analyzed and they significantly affected the digestion of flavonoids, especially quercetin and hesperetin. In the stomach and duodenum, those compounds were metabolized to a limited extent. On the other hand, the intestinal microbiota affected the phenolic acids to a lesser extent.