U 4+ is used to probe the incommensurate structure of ThBr 4 via absorption and magnetic circular dichroism (MCD) measurements. Below T c =95 K, the crystal field experienced by U 4+ ions is either of D 2d or D 2 symmetry. This can be simulated by varying a phase angle φ form 0(D 2d ) to ±π/2(D 2 ). One very neat advantage of MCD over absorption is that the D 2d centres show up very clearly in the former due to the occurrence of a derivative-like singularity. We present a model to simulate absorption, Zeeman and MCD profiles. It accounts satisfactorily for most of our data and we arrive at the following main conclusions: (i) it is probable that the phase is truly incommensurate since the lower limit of the number of sites is found to be 80; (ii) for a low concentration of U 4+ (2×10 −4 ), we found no evidence of a pinning of the incommensurate modulation by the impurities. MCD provides also a definite proof that the ground state of U 4+ in ThBr 4 is Γ 4 with a D 2d symmetry. Finally, we determine the Lande factors of five excited states, our findings being in fair agreement with the results of recent crystal field calculations Au-dessous de T c =95 K, la symetrie du site de l'ion sonde peut etre simulee en faisant varier un angle de phase entre 0(symetrie D 2d ) et ±π/2(symetrie D 2 ). Presence d'une singularite pour les centres D 2d dans les mesures de dichroisme circulaire magnetique. Interpretation par une phase vraiment commensurable (limite inferieure du nombre de sites differents: 80). Absence d'ancrage de l'onde incommensurable par les impuretes pour une concentration inferieure a 2•10 −4 . Etat fondamental Γ 4 en symetrie D 2d de l'ion U 4+ . Facteur de Lande de 5 niveaux excites
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