Catalytic β-stereoselective epoxidation of Δ5-unsaturated steroid derivatives has been effected by a novel ruthenium(II) bioxazoline complex under aerobic conditions. The reactions are regio- and stereoselective. The reaction conditions provide the corresponding 5β,6β-epoxides with high degree of stereoselectivity (88-96%) in very good yields, while oxidation of steroid derivatives with peracids leads to 5α,6α-epoxides as the major products. The overall conformation of the steroid nucleus is nearly planar in the cholesteryl ester, while it is bent at the junction between the rings A and B in the 5β,6β-epoxide. This change from pseudo-trans- to cis-stereochemistry of the A-B ring junction provides more room for the catalyst to approach from the β-face of the steroidal skeleton. © 1998 American Chemical Society.