Kinetics of reduction of toluidine blue (hereafter referred to as TB+) by sodium dithionite in aqueous hydrochloric acid medium has been studied spectroscopically at λ = 600 nm. The order of the reaction with respect to TB+ and dithionite ion was found to be one and 1.39 respectively. The rate of the reaction increased with increase in [H+]. Variation of the ionic strength and dielectric constant of the medium altered the rate of the reaction, addition anions (X = SO4 2-, NO3 -) to the reaction mixture decreased the rate of the reaction. Spectroscopic and kinetic investigation showed no sign of intermediate complex formation; free radical polymerisation test showed the presence of free radicals. A plausible mechanism which accommodates all the experimental data was proposed. The rate law for the reaction conforms to the equation: Rate = k4K2[TB+] [S2O4 2-][H+] + k5 k3K1 ½[TB+] [S2O4 2-]½[H+]