Expressions are derived for rate constants and activation parameters of electrochemical reactions that are accompanied by a change in spin-state. Predictions based on an electrochemical scheme of squares are compared with experimental results for [M(tacn)(2)](3+/2+) (M = Fe, Co, Ni, Ru; tacn = 1,4,7-triazacyclononane) and [Fe(pzb)(2)](+/0) (pzb(-) = hydrotris(pyrazol-1-yl)borate [HB(pz)(3-)], tetrakis (pyrazol-1-yl)borate [B(pz)(4-)] or hydrotris(3,5-dimethylpyrazol-1-yl)borate [HB(Me(2)pz)(3-)]) redox couples. The electrochemical rate constants, (k(s,h))(obs), of [Co(tacn)(2)](3+/2+) and [Fe{HB(Me(2)pz)(3)}(2)](+/0) are decreased by the presence of spin-exchange. Electrochemical enthalpies (DeltaH(obs)(double dagger)) and entropies (DeltaS(obs)(double dagger)) of activation, determined from the temperature dependence of (k(s,h))(obs), under nonisothermal cell conditions, provide evidence regarding the mechanismof these reactions. [Co(tacn)(2)](3+/2+), [Fe(tacn)(2)] (3+/2+), and [Fe{HB(Me(2)pz)(3)}(2)](+/0) exhibit values of DeltaH(obs)(double dagger) and DeltaS(obs)(double dagger) that are larger than anticipated for simple electron-transfer, consistent with the interpretation that these reactions occur by coupled rather than concerted electron- transfer and spin-exchange and proceed through a high-spin intermediate in the M(III) oxidation state. Spin-exchange is viewed as a rapidly established, temperaturedependent equilibrium, the thermodynamic consequences of which are reflected in the electrochemical activation parameters.