In this report, we provide an x-ray magnetic circular dichroism XMCD study for 100 nm thick epitaxial magnetite Fe 3 O 4 films on MgO 001 and Al 2 O 3 0001 substrates. For XMCD, we recorded the surface sensitive total electron yield and the bulk sensitive transmission spectra. From the analysis of the XMCD data, we find an increased Fe spin moment 10% larger at the surface of the film on MgO 100 with respect to the corresponding bulk value of the film. Surface and bulk spin moments of the film on Al 2 O 3 0001 are almost equal. For both films, the bulk orbital to spin moments ratio increases from zero at 70 K to 0.03–0.04 at 300 K. For Fe 3 O 4 / MgO 001, the surface orbital to spin moment ratio behaves similarly to the bulk value, while the orbital to spin moments ratio is increased at the Fe 3 O 4 / Al 2 O 3 0001 surface to 0.06. The observed differences between films grown on MgO and Al 2 O 3 are explained within the framework of differences in mismatch strain experienced by the films. © 2008 American Institute of Physics.