We studied whether dietary non-digestible oligosaccharides (NDOs) affected pH and volatile fatty acids (VFAs) in gastrointestinal contents and in portal plasma of young pigs. Five groups of five 57-day-old pigs received for 44 days either a corn-based control diet or this diet with 7.5 or 15 g/kg fructooligosaccharides (FOSs, Raftilose P95?) or the control diet with 10 or 20 g/kg transgalactooligosaccharides (TOSs, Oligostroop?). The pigs weighed on average 45.5±1.3 kg during dissection, which took place 3 h after feeding. Dietary NDOs tended to lower the pH of the stomach content from 4.5 to 4.2 (P=0.06). Pigs fed the high TOS diet had more caecal VFAs than the control pigs (30.4 vs. 15.6 mmol, P<0.05). Compared to TOS-fed pigs, FOS-fed pigs had a higher proximal colon pH (6.5 vs. 6.2, P<0.01), lower proximal colon VFA concentration (131 vs. 166 mmol/l, P<0.01) and lower portal VFA concentration (0.9 vs. 1.6 mmol/l, P<0.05), with the control pigs being intermediate. However, the amount of colonic VFAs was similar across diets (~40 mmol). The results support the view that dietary FOSs and TOSs may have different effects on fermentation characteristics of gut contents of pigs.