We present a numerical study of the applicability of the sum rule analysis used, in X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS), to evaluate the atomic spin moment of 3d transition metals. Magnetic circular dichroism (MCD) is present both in XAS and RIXS at the L-2.3 absorption edges, although the two sum rules presented here assume different experimental setups: in XAS-MCD the magnetic field is parallel to the circularly polarised incident photons, in RIXS-MCD the two are orthogonal (perpendicular geometry). For Mn2+ we explore XAS- and RIXS-MCD by performing atomic model calculation in the single ion limit with inclusion of the appropriate crystal and magnetic fields. The comparison of the XAS-MCD spectra with those measured from a MnFe2O4 sample is used to fix the calculation parameters. The applicability of the XAS- and RIXS-MCD sum rules to Mn2+ is numerically checked within the chosen calculation model. (C) 2002 Elsevier Science B.V. All rights reserved.