Chemical Laboratories, Holkar Science College, Indore, (India) Chemical Laboratories, Government College, Dhar, (India) Manuscript received 25 July 1978, revised 24 November 1978. accepted 12 March 1979 TI(III) oxidation of lactic acid, catalysed by manganous ions in aqueous acetic acid, has been found to induce the polymerization of methyl methacrylate showing the inter­vention of organic free radical of the type HOCHR. The observation indicates that TI(III) reacts as one electron oxidant in the oxidation of lactic acid with intermediate formation of TI(II). The catalytic function may be ascribed to equilibria, TI(III)+ Mn(II)L\(\rightleftharpoons\)TI(II)+Mn(III)L in which the lactic acid functions as a ligand. The reaction with respect to each, oxidant as well as organic substrate has been found to obey first order kinetics. Thallic acetate has been shown to be the active oxidant in aqueous acetic acid medium. The values of thermodynamic parameters, viz. entropy of activation, heat of activa­tion, free energy of activation, energy of activation and frequency factor have been evaluated as, ∆ S = 23.33 e.u., ∆ H=30.1 K cals/mol, ∆ F=23.0 K cals/mol., ∆ E= 30.84 K cals/mol and PZ =8.35 x 1017 min-1, respectively. A mechanism is proposed to account for the observed stoichiometry, kinetic para­meters and other experimental observations.