Conversion of [Co 3 (μ 3 -CR'){μ-(AsMe 2 ) 2 O}(CO) 7 ] [R'=Cl, Me] and [Co 2 {μ-C 2 (CO 2 Me) 2 }{μ-(AsMe 2 ) 2 O}(CO) 4 ] to the corresponding bis-dimethylarsinesulfide-bridged complexes [Co 3 (μ 3 -CR'){μ-(AsMe 2 ) 2 S}(CO) 7 ] [R'=Cl (1a), Me (1b)] and [Co 2 {μ-C 2 (CO 2 Me) 2 }{μ-(AsMe 2 ) 2 S}(CO) 4 ] (2) can be achieved on their treatment with H 2 S (with the elimination of H 2 O); this process is reversible. The related bis-diphenylarsinesulfide-bridged complexes, [Co 3 (μ 3 -CR'){μ-(AsPh 2 ) 2 S}(CO) 7 ] [R'=Cl (3a), Me (3b)] and [Co 2 (R'CCR''){μ-(AsPh 2 ) 2 S}(CO) 4 ] [R'=R''=CO 2 Me (4a), Ph (4b)] can be obtained in moderate yield by the direct reaction of (Ph 2 As) 2 S with [Co 3 (μ 3 -CR')(CO) 9 ] [R'=Cl, Me] and [Co 2 (R'CCR'')(CO) 6 ] [R'=R''=CO 2 Me, Ph], respectively. The triphenylarsine mono-substituted complex [Co 2 {μ-C 2 (CO 2 Me) 2 }(CO) 5 (AsPh 3 )] (5) is also isolated as a product in the reaction leading to 4a. Thermolysis of 3a yields [Co 3 (μ 3 -S){μ-(AsPh 2 ) 2 O}{μ-(AsPh 2 S)}(CO) 7 ] (6) in low yield while the related thioxo-capped complexes [Co 3 (μ 3 -S){μ-(AsMe 2 ) 2 X}(μ-AsMe 2 S)(CO) 5 ] [X=S (7), O (8)] can be obtained in higher yield by thermolysis of [Co 3 (μ-CMe){μ-(AsMe 2 ) 2 O}(CO) 7 ] in the presence of H 2 S. Single crystal X-ray diffraction studies have been performed on 2, 4a, 4b and 8.