Second-order rate constants k(12)(obsd) measured at 25 degrees C in acetonitrile by stopped-flow for 47 electron transfer (ET) reactions among ten tetraalkylhydrazines, four ferrocene derivatives, and three p-phenylenediamine derivatives are discussed. Marcus’s adiabatic cross rate formula k(12)(calcd) (k(11)k(22)k(12)f(12))(1/2), ln f(12) = (ln K-12)(2)/4 ln(k(11)k(22)/Z(2)) works well to correlate these data. When all k(12)(obsd) values are simultaneously fitted to this relationship, best-fit self-exchange rate constants, k(ii)(fit), are obtained that allow remarkably accurate calculation of k(12)(obsd); k(12)(obsd)/k(12)’(calcd) is in the range of 0.55-1.94 for all 47 reactions. The average Delta Delta G(ij)(not equal) between observed activation free energy and that calculated using k(ii)(fit) is 0.13 kcal/mol. Simulations using Jortner vibronic coupling theory to calculate k(12) using parameters which produce the wide range of k(ii) values observed predict that Marcus’s formula should be followed even when V is as low as 0.1 kcal/mol, in the weakly nonadiabatic region. Tetracyclohexylhydrazine has a higher k(ii) than tetraisopropylhydrazine by a factor of ca. 10. Replacing the dimethylamino groups of tetramethyl-p-phenylenediamine by 9-azabicyclo[3.3.1]nonyl groups has little effect on k(ii), demonstrating that conformations which have high intermolecular aromatic ring overlap are not necessary for large ET rate constants. Replacing a gamma CH2 group of a 9-azabicyclo[3.3.1]nonyl group by a carbonyl group lowers k(ii) by a factor of 17 for the doubly substituted hydrazine and by considerably less for the doubly substituted p-phenylenediamine.