High performance capillary electrophoresis (HPCE) as well as high performance liquid chromatography-mass spectrometry (HPLC-MS) have been applied to the separation, identification and quantification of the tumor-inhibiting ruthenium compounds HIm trans-[RuCl4(im)2] (im = imidazole) and HInd trans-[RuCl4(ind)2] (ind = indazole) and their hydrolysis products. The half-lives for the hydrolytic decomposition of the Ru(III) compounds were determined by monitoring the relative decrease of the original complex anion under different conditions by means of capillary electrophoresis. The decomposition follows pseudo-first-order kinetics. The rate constants in water at 25 degrees C are 1.102 +/- 0.091 x 10(-5) s-1 for HIm trans-[RuCl4(im)2] and 0.395 +/- 0.014 x 10(-5) s-1 for HInd trans-[RuCl4(ind)2]. About 8% of HIm trans-[RuCl4(im)2] but only about 2% of HInd trans-[RuCl4(ind)2] were hydrolyzed after 1 h at room temperature. Whereas the hydrolysis rate of the imidazole complex is independent of the pH value, the indazole complex hydrolyzes much faster at higher pH. The half-life of HInd trans-[RuCl4(ind)2] in phosphate buffer at pH 6.0 and 37 degrees C is 5.4 h, whereas it is less than 0.5 h at pH 7.4. In contrast to the imidazole complex, where no dependence on the buffer system was observed, hydrolysis of the indazole complex is even faster if a buffer containing hydrogen carbonate is used. The formation of [RuCl2(H2O)2(im)2]+ could be demonstrated by HPLC-MS measurements. In the case of the indazole complex, a release of the indazole ligands results in the formation of [RuCl4(H2O)2]-.