The tetranuclear ruthenium clusters [Ru4(CO)6(μ-CO)3(μ4-S) 2{C(NHR)2}2] 1 (R = Pri) and 3 (R = Et) and [Ru4(CO)7(μ-CO)3(μ4-S) 2{C(NHR)2}] 2 (R = Pri) and 4 (R = Et) were obtained from the thermal reaction of [Ru3(CO)12] with thioureas SC(NHR)2 (R = Pri or Et). In the case of diethylthiourea, a trinuclear cluster [Ru3(μ-H)(CO)9(μ3-NEtCSNHEt)] 5 was isolated in addition to 3 and 4. The single-crystal X-ray structure analyses of 1 and 2 reveal a pseudo-octahedral Ru4S2 skeleton with three μ-carbonyl bridges in the Ru4 plane; 1 has two terminal diaminocarbene ligands and six terminal carbonyls, whereas there is only one terminal diaminocarbene ligand but seven terminal carbonyls in 2.