Aquation of the anticancer complex trans-[H₂im][RuCl₄(Him)₂]1(Him = imidazole) has been studied in D₂O at a range of pH*(meter reading) values between 2.4 and 10 by observation of paramagnetically shifted ¹H NMR resonances. At 310 K. pH* 5.6, the half-life was 3.4 h and three products were detected, assigned as mono- and di-aqua species. The rate was similar at low pH*, but the reaction followed a different course at high pH*. Aquation was also studied by separation of the products on a reversed-phase column using HPLC, by conductivity measurements, and by EPR spectroscopy. Slow oxidation of the complex to Ru^IV appeared to take place in perchlorate, phosphate or acetate solutions. Cyclic voltammetric studies showed that reduction of 1 to Ru^II(E_m–0.14 V versus the normal hydrogen electrode, pH 7) was accompanied by the uptake of two protons with associated pK_a values of 6.26 and 3.87. Both aquation and the more favourable reduction of 1 at low pH may play a role in the biological activity of the complex.