Beside the now well known self-absorption effect, several phenomena related to the multiplet structure of the intermediate state the occur which will render the X-ray fluorescence different from the true absorption result on 3d transition metals at the L edge and on the M4,5 edges of rare earths. Special selection rules of the radiative de-excitation process may play an important role there. We have measured the absorption coefficient of a thin film of thulium deposited onto an aluminum foil, at room temperature, through the simultaneous detection of the transmission, 160 eV bandwidth fluorescence yield and total electron yield. While transmission and electron yield results are very similar, as expected for a very thin sample, the fluorescence yield spectrum is definitely different. Theoretically, the resonant X-ray fluorescence spectrum was calculated using an atomic model, and then integrated over the emitted energy, to predict the fluorescence yield spectrum. A very good agreement is obtained between the theory and experiment.