A general mechanism for Ru(III) catalysed oxidation of L(+) arabinose (Ara) and D(+) galactose ( Gal) in aqueous alkaline medium is proposed. The most reactive species of the catalyst was found to be [RuCl3(OH)(2)H2O](2)(-). The reactions exhibit pseudo first order kinetics with respect to Ru(III) and are first order with respect to lower [IO4-] and OH-, but tend towards zeroth order to both higher [IO4-] and [OH-]. The reactions are zero order with respect to each aldoses. Positive effect of Cl- on the rate of reaction is also evident in the oxidation of both reducing sugars. A mechanism, involving the interaction of the reactive species of Ru(III) and [IO4-] leading to the formation of a complex, which in the rate determining step decomposes into another complex with the release of Cl- ion, is proposed and the rate law is derived.