The kinetics of electron-transfer reactions of some Co(III) complexes with Ru(XH3)2+ (outer sphere), Cr(HtOj6~+ (inner'sphere), and V(F&O)2+ (inner sphere) have been studied in the presence of varying concentrations of the sodium salt of poly(methacryloxyethylsulfonic acid), (PMES), using the stopped-flow technique. A large acceieration of reaction rates was observed with Ru(XH,),s+ and V(HsO) &z+; with acceleration factors up to 20,099. The accelerated rates of ekctron transfer show a nonlinear dependence on polyelectrolyte concentrations: fust, increasing with increasing poIyeIectroIyte concentration then reaching a maximum and subsequently decreasing. The magnitude of acceIeration depends strongIy on the charge of oxidants, being much huger for dipositive than for unipositive ions. AcceIeration specificity is shown by the reduction of cis-Co(en):Cb+ by Ru(XH&i=*, in which case the acceleration factor is much higher in the presence of PMES than in the case of the corresponding tram isomer. The accekration of the reaction of &-Co(en)&CF+ nith Ru(XH~)~J~* by the poIyelectrol~-te is practically equal to the ac-ceIeration of the reduction of this complex by V(H:O)2*, whereas the reduction of &s-Co(en)&H3C+ by Ru(XH&~+ is more strongly ac-ceIerat.ed than the reduction by V(RzO)a*c_ Activation energies have been determined for some of the reactions and have been interpreted in terms of a multiple equilibrium reaction scheme_ At a given concentration of reactants and poIyeIectroIq;te, the acceleration factor increases with decreasing temperature and, further, the concentration of poly-