The objective was to quantify the vitamin A equivalency of β-carotene in two diets using a dual-isotope dilution technique and the apparent β-carotene absorption as measured by the oral–faecal balance technique. Seventeen healthy adults with an ileostomy completed the 4-week diet-controlled, cross-over intervention study. Each subject followed both diets for 2 weeks: a diet containing vegetables low in β-carotene content with supplemental β-carotene in salad dressing oil (‘oil diet’; mean β-carotene intake 3·1 mg/d) and a diet containing vegetables and fruits high in β-carotene content (‘mixed diet’; mean β-carotene intake 7·6 mg/d). Daily each subject consumed a mean of 190 μg [13C10]β-carotene and 195 μg [13C10]retinyl palmitate in oil capsules. The vitamin A equivalency of β-carotene was calculated as the dose-corrected ratio of [13C5]retinol to [13C10]retinol in serum. Apparent absorption of β-carotene was determined with oral–faecal balance. Isotopic data quantified a vitamin A equivalency of [13C10]β-carotene in oil of 3·6:1 (95 % CI 2·8, 4·6) regardless of dietary matrices differences. The apparent absorption of (labelled and dietary) β-carotene from the ‘oil diet’ (30 %) was 1·9-fold higher than from the ‘mixed diet’ (16 %). This extrinsic labelling technique can measure precisely the vitamin A equivalency of β-carotene in oil capsules, but it does not represent the effect of different dietary matrices.