Indazolium trans-[tetrachloridobis(1H-indazole)ruthenate(III)] (KP1019) and its Na + analogue (KP1339) are two of the most prominent non-platinum antitumor metal complexes currently undergoing clinical trials. After intravenous administration, they are known to bind to human serum albumin (HSA) in a non-covalent manner. In order to elucidate their HSA binding sites, displacement reactions with the established site markers warfarin and dansylglycine as well as bilirubin were monitored by spectrofluorimetry, ultrafiltration−UV-vis spectrophotometry and/or capillary zone electrophoresis. Conditional stability constants for the binding of KP1019 and KP1339 to sites I and II of HSA were determined, indicating that both Ru(III) compounds bind into both sites with moderately to strong affinity (logK 1 ' = 5.3-5.8). No preference for either binding site was found and similar results were obtained for both metal complexes, demonstrating low influence of the counter ion on the binding event.