Department of Post-graduate Studies and Research in Chemistry, University of Mysore, Manasa Gangotri, Mysore 570006, India. Manuscript received 12 July 1976, accepted 10 February 1977. Mepazine hydrochloride forms a red coloured complex with ruthenium(III) at room temperature in acid medium. The red complex exhibits absorption maxi­mum at 512-516 am. Beer's law is valid over the concentration ranges 0.40-12.8 ppm in sulphuric acid and 0.32-6.4 ppm in hydrochloric acid. The Sandell's sensi­tivity of the reaction is 0,022 \(\mu\)g/cm2. in H2SO4 and 0.015 µg cm2 in HCI. The respective molar absorptivity is 4.48 x 103 1 mol-1cm-1 in H2SO4 and 6.7 x 103 1 mol-1 cm-1 in HCI. The composition study by molar ratio method indicates that the ruthenium-MH complex has a ratio of 1 : 2.