The aim of this trial was to investigate the effects that dietary tannins have on lamb intramuscular fatty acids. Twenty-seven lambs were divided into three homogeneous groups: control group, receiving commercial concentrate based on maize; tannin group, fed a diet based on carob pulp (45% as fed basis); PEG group, receiving the same diet as the latter with addition of 42g/kg of polyethylene glycol (PEG, a binding agent that eliminates the effects of condensed tannins). The duration of the trial was 45 d. Intramuscular fatty acids were measured in the longissimus dorsi muscle. The isomer cis-9 trans-11 of linoleic acid (conjugated linoleic acid or CLA) and linolenic acid were higher in the longissimus muscle fat from animals fed the control diet compared to the other groups (P<0.0005); these fatty acids were higher in the fat from animals fed the carob diet supplemented with PEG compared to those fed the same diet without PEG (P<0.05). trans-Vaccenic acid (C18:1 trans-11) was found at higher concentration in fat from control and PEG lambs compared to tannin lambs (P<0.01); the CLA/C18:1 trans-11 ratio was lower in lambs fed control and PEG diets than in tannin-fed animals (P<0.05).