Duration of soybean oil (SBO) supplementation needed to enhance carcass conjugated linoleic acid (CIA) and trans-vaccenic (TVA) content was examined using 96 beef steers (293.6 +/- 3.9 kg) fed a 78% corn-based diet supplemented with SBO for 0, 77, 137, or 189 days before slaughter. Duration of SBO supplementation had no effect (P >= 0.15) on animal performance or carcass traits, nor (P >= 0.15). total, total Saturated, or total polyunsaturated fatty acids of Longissimus dorsi (LD). Concentrations of CLA in LD were not affected (P >= 0.18) by SBO supplementation. Concentrations of monounsaturated fatty acids (MUFA) decreased linearly (P = 0.03) in LD, whereas TVA increased (P = 0.04) in adipose tissue and tended (P = 0.07) to increase in LD with increasing duration of SBO supplementation. Supplementing SBO to a concentrate-based diet may enhance TVA without impacting CLA, while reducing the MUFA content of lean beef. Published by Elsevier Ltd.