Non-starch polysaccharides (NSP) reduce digestibility, but relative contributions of their functional properties viscosity (V) and fermentability (F) are not known. Thus, 8 ileal-cannulated pigs were fed 4 diets based on cornstarch and casein and supplemented with purified NSP sources, either 5% low F, low V cellulose (CEL), low F. high V carboxymethylcellulose (CMC), high F, low V oat beta-glucan (LG), or high F. high V oat beta-glucan (HG) in a double 4 x 4 Latin square. Apparent Heal (AID) and total tract digestibility (ATTD) were calculated using TiO2 as indicator. The AID of energy, crude protein (CP) and dry matter (DM) was highest (P<0.001) for high viscous CMC. The ATTD of energy and DM was highest for CMC (P<0.05) and MID of CP did not differ among diets. The AID and ATTD of ash was only positive for CMC (P<0.05) indicating secretion of minerals with other NSP. Post-ileal DM digestibility, an indicator of fermentation, was highest (P<0.0001) for CEL and HG. The relationship of AID of energy to post-ileal DM digestibility was stronger (R-2=0.85; P<0.001) than the relation with digesta V (R-2=0.45; P<0.001). In conclusion, negative effects of NSP on AID of energy and CP are explained better by increased fermentability than by increased digesta viscosity. (C) 2010 Elsevier B.V. All rights reserved.