A structure−activity relationship study on triosmium clusters of the general formula Os 3 (μ-H)(μ-SC 6 H 4 X)(CO) 10 (2-X, where X = o-, m-or p-NH 2 ; o-, m-or p-OH; p-H, -Br, -NO 2 , -COOH or -CH 2 COOH) show that their antiproliferative activity is through the cluster core, and the nature and position of the phenyl ring substituent X shows a significant impact on the activity. Clusters with an electron-withdrawing group are more reactive but are thus quickly consumed through reaction with serum, while those with an electrondonating group persists sufficiently to enter the cells and result in higher antiproliferative activity. Interestingly, m-substituted clusters and those with lipophilicity >6.0 also exhibit higher antiproliferative activity. In contrast, o-substituted clusters capable of intramolecular hydrogen bonding have lower cytotoxicity. The cluster 2-m-OH, with higher antiproliferative activity and lower reactivity with serum, is a potential lead compound for further mode of action studies.