Dietary flavonoids are poorly absorbed from the gastrointestinal tract. Colonic bacteria convert flavonoids into smaller phenolic acids (PA), which can be absorbed into the circulation and may contribute to the chemopreventive activity of the parent compounds. The purpose of our study was to determine whether flavonoids from green and black tea (GT, BT), citrus fruit with rutin (CF+R) and soy (S) supplements exposed to the same conditions in a dynamic in vitro model of the colon (TIM-2) will form the same phenolic acid products of microbial metabolism. About 600 mg of flavonoids from GT, BT, CF+R and S extracts were infused at t = 0 and 12 h into the TIM-2. Samples from the lumen and dialysate were collected at t = 0,4,8,12,16,24 and 28h. The flavonoid and PA concentrations were measured by HPLC and GC-MS. GT, BT, and CF+R formed 3-methoxy-4-hydroxyphenylacetic acid (3M4HPAA), 4-hydroxyphenyl acetic acid (4HPAA), 3,4-dihydroxyphenylacetic acid (3,4DHPAA), and 3-(3-hydroxyphenyl) propionic acid (3,3HPPA). BT flavonoids were also metabolized to 2,4,6-trihydroxybenzoic acid (2,4,6THBA) and CF+R flavonoids to 3-(4-hydroxy-3-methoxyphenyl) propionic acid (3,4H3MPPA), 3-hydroxyphenyl acetic acid (3HPAA) and a small amount of hippuric acid. After S infusion, we found 3M4HPAA and 4HPAA only. Among these phenolic acids, only 3,4DHPAA exhibited antiproliferative activity in prostate and colon cancer cells. 3,4DHPAA was significantly (P < 0.005) more inhibitory in colon cancer cells (HCT116) compared with an immortalized normal intestinal epithelial cell line (IEC6). In summary, fermentation by intestinal microbes of GT, BT, C+R, and S flavonoids resulted in the conversion to the same major phenolic acids. © 2006 American Society for Nutrition.