Magnetic properties influenced by interfaces in ultrathin Co/Ge(I 0 0) and Co/Ge(I 1 1) films with thickness below 28 monolayers (ML) have been studied using the surface magneto-optic Kerr effect (SMOKE) technique. In both systems, the nonferromagnetic layer, as an interface between Co and Ge, plays an important role during annealing. In general, ultrathin Co films with fixed total thickness but fabricated at different temperatures on the same substrate, their Kerr hysteresis loops disappear roughly at the same temperature. This suggests that the thickness of the interfacial layer could inversely prevent the diffusion between Co and Ge substrate. From the annealing studies for both systems with total film thickness of 28 monolayers, we have found that Kerr signal disappears at 375 K for Co/Ge(I 1 1) and 425 K for Co/Ge(I 0 0) films. This suggests that Co/Ge(I 1 1) films possess a lower thermal stability than that of the Co/Ge(l 0 0) films. Our experimental data could be explained by different interfacial condition between Ge(I 0 0) and Ge(I 1 1), the different onset of interdiffusion, and the surface structure condition of Ge(I 0 0) and Ge(I 1 1). (C) 2003 Elsevier Science B.V. All rights reserved.