Ultrathin layers (2-6 atomic monolayers) of the body-centered cubic (bcc) Co phase have been investigated by X-ray diffraction. Single-crystalline MgO (0 0 1) substrates and thin layers of Fe have been used to stabilize a (0 0 1) oriented single-crystalline bcc Co structure. The Co-on-Fe (Co deposited on Fe) interfaces in the Fe/Co superlattices are found to be abrupt up to a substrate temperature of about 500degreesC whereas the Fe-on-Co (Fe deposited on Co) interfaces become more diffuse at 350degreesC due to interdiffusion of Fe into the Co layers. The structural quality of the Fe/Co superlattices exhibits a strong temperature dependence. A marked decrease of the linewidth of the Fe/Co (0 0 2) Bragg peak with increased growth temperature is found for superlattices with high Co content. The decrease is attributed to the interdiffusion of Fe into the bee Co layers. (C) 2003 Elsevier Science B.V. All rights reserved.