Detailed ab initio calculations were performed on two structurally different cerium(III) single-molecule magnets (SMMs) to probe the origin of magnetic anisotropy and to understand the mechanism of magnetic relaxations. The complexes [CeIII{ZnII(L)}2(MeOH)]BPh4 (1) and [Li(dme)3][CeIII(cot′′)2] (1; L=N,N,O,O-tetradentate Schiff base ligand; 2; DME=dimethoxyethane, COT′′=1,4-bis(trimethylsilyl)cyclooctatetraenyldianion), which are reported to be zero-field and field-induced SMMs with effective barrier heights of 21.2 and 30K respectively, were chosen as examples. CASSCF+RASSI/SINGLE-ANISO calculations unequivocally suggest that mJ|±5/2 and |±1/2 are the ground states for complexes 1 and 2, respectively. The origin of these differences is rooted back to the nature of the ligand field and the symmetry around the cerium(III) ions. Ab initio magnetisation blockade barriers constructed for complexes 1 and 2 expose a contrasting energy-level pattern with significant quantum tunnelling of magnetisation between the ground state Kramers doublet in complex 2. Calculations performed on several model complexes stress the need for a suitable ligand environment and high symmetry around the cerium(III) ions to obtain a large effective barrier. CeIII Jiggling! Some CeIII mononuclear complexes exhibit single-molecule magnetic (SMM) behaviour, whereas others do not (see figure). This intriguing point has been investigated by using ab initio calculations and the role of ligand field and symmetry in dictating SMM characteristics is highlighted. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.