The aim of this work was to differentiate the feed received by Iberian swine during fattening (acorns, feed) and their breed (Iberian or White) using analysis of the stable isotopes of carbon (δ(13)C) and sulphur (δ(34)S) in liver tissue samples. The results obtained in the determination of δ(34)S, using a procedure in which organic and inorganic sulphur are converted into BaSO(4) and the procedure that measures δ(34)S in samples of dried ground liver tissue were compared. Joint analysis of carbon (δ(13)C) and sulphur (δ(34)S) permits the differentiation of swine of different breeds receiving different diets (acorns or feed).