The mixed-valent compounds [Ru 2 (μ-O 2 CCH 3 ) 4 (4,4'-dipy)](PF 6 ) (1), [Ru 2 (μ-O 2 CCH 3 ) 4 (4,4'-dipy)] (BPh 4 ) . H 2 O (2) and [Ru 2 (μ-O 2 CCH 3 ) 4 (dabco)] (PF 6 ) (3) (4,4'-dipy=4,4'-dipyridine and dabco = 1,4-diazabicyclooctane) were synthesized and characterized by IR spectroscopy, elemental analysis and variable temperature magnetic susceptibility. The magnetic measurements in the range 4-305 K show a significant drop in μ e f f at low temperatures which can be accounted for by a large zero-field splitting (ZFS) and weak intermolecular antiferromagnetic exchange. The model previously employed by Cukiernik et al. (Inorg. Chim. Acta, 215 (1994) 203) was used and found to be successful in fitting the magnetic data. The intermolecular coupling constants, zJ, were determined to be -0.996(1), -0.653(1) and -0.593(1) cm - 1 for complexes 1, 2 and 3 respectively.