Five new dinuclear MnIII compounds [{Mn(bpy)(H2O)}2(μ-4-RC6H4COO)2(μ-O)](NO3)2 [R = Me (1), F (2), CF3 (3), MeO (4) and tBu (5); bpy = 2,2′-bipyridine] have been synthesized; compounds 1–3 have been characterized by X-ray diffraction. Moreover, a few crystals of the MnIII compound[{Mn(bpy)(H2O)}(μ-4-FC6H4COO)(μ-O){Mn(NO3)(bpy)(H2O)}](NO3)2 (6) have been obtained. Variable-temperature magnetic susceptibility data of 1–5 were recorded; all compounds show ferromagnetic behaviour with J = 1.48 cm–1 for 1, 1.44 cm–1 for 2, 5.66 cm–1 for 3, 3.22 cm–1 for 4 and 2.68 cm–1 for 5 (H = –JS1·S2); and |DMn| = 1.5 cm–1 and |EMn| = 0.5 cm–1 for all compounds except for 2, for which these values are 2.85 and 0.7 cm–1, respectively. The coordination octahedron around the MnIII ions shows a rhombic distortion due to the coordination of a water molecule. This kind of distortion and the R group of the benzoato–derivative bridge in a para position favour ferromagnetic behaviour. Moreover, the presence of an electron-withdrawing substituent increases this magnetic character. All compounds show catalase activity with a conversion range between 20 and 55 % of the initial H2O2 within 10 min. Compounds 4, 5 and 3 with an electron-donating R group are more efficient than 2, 3 with electron-withdrawing groups.