Seven experimental diets were formulated: basal diet (A) to which 19 g/kg (B), 38 g/kg (C), 58 g/kg (D) and 76 g/kg (E) vegetable oil blend was added. Diets F and G were barley- and maize-based diets. Study 1 used 14 pigs to determine total tract digestibility and digestible energy (DE) content. The basal diet contained: g/kg: barley 250, wheat 160, maize germ 65, maize gluten 50, maize gluten feed 50, wheat pollard 150, rapeseed 75, soyabean meal 163 and other ingredients 37. Study 2 used 1232 pigs on a performance trial. Digestibility coefficients were higher (P<0.001) for the cereal-based diets. Liveweight gain was highest (P<0.05) for pigs offered cereal-based diets but feed conversion efficiency (FCR) was similar compared with values obtained from pigs offered by-product-based diets plus oil. There was a significant (P<0.001) linear effect on FCR but not for DE:gain, with increasing oil addition. In conclusion, supplementation of by-product-based diets with oil increased DE to similar levels as cereal-based diets. However, cereal-based diets resulted in higher levels of DE intake and liveweight gain.