The magnetism of the salts [Fe mephen3]X2 (X = I-, ClO4-, BF4-, PF6-; mephen = 2-methyl- 1,l0-phenanthroline) has been investigated over the temperature range c. 100-300�K. The compounds exhibit anomalous behaviour, and this has been qualitatively interpreted as being the result of the presence of nearly equi-energetic ground terms 1A1 and 5T2 of the complexed iron(11) ion, a thermal equilibrium between these terms causing the magnetic moments to be markedly temperature dependent. This behaviour is apparently the result of the sterie hindrance to the coordination of an iron(11) ion induced by the presence of the methyl group in the ligand and the consequent lowering of the effective strength of the ligand field. The colour of the salts varies with temperature, and this has been discussed in terms of π-bonding between the iron(11) ion and the ligand.