Based on the potential benefits to long-term human health there is interest in developing sustainable nutritional strategies for reducing saturated and increasing specific unsaturated fatty acids in ruminant milk. The impact of plant oil supplements to diets containing different forages on caprine milk fatty acid composition was examined in two experiments using twenty-seven Alpine goats in replicated 3 × 3 Latin squares with 28 d experimental periods. Treatments comprised of no oil (control) or 130 g/d of sunflower-seed oil (SO) or linseed oil (LO) supplements added to diets based on grass hay (H; experiment 1) or maize silage (M; experiment 2). Milk fat content was enhanced (P < 0·01) on HSO, HLO and MLO compared with the corresponding H or M control diets, resulting in 17, 15 and 14 % increases in milk fat secretion, respectively. For both experiments, plant oils decreased (P < 0·05) milk 10 : 0–16 : 0 and odd- and branched-chain fatty acid content and increased 18 : 0, trans-Δ6–9,11–14,16-18 : 1 (and their corresponding Δ-9 desaturase products), trans-7, trans-9-conjugated linoleic acid (CLA), trans-9, trans-11-CLA and trans-8, cis-10-CLA concentrations. Alterations in the distribution of cis-18 : 1, trans-18 : 1, -18 : 2 and CLA isomers in milk fat were related to plant oil composition and forage in the diet. In conclusion, plant oils represent an effective strategy for altering the fatty acid composition of caprine milk, with evidence that the basal diet is an important determinant of ruminal unsaturated fatty acid metabolism in the goat.