Alkyl substituent effects in gas-phase and solution redox thermochemistry have been studied for a series of ruthenium coordination complexes. The gas-phase free energies of ionization (Delta G(i) degrees) and electron attachment (Delta G(a) degrees) are compared to electrochemical oxidation and reduction half-wave potentials (E(1/2)) for six ruthenium tris-(beta-diketonate) complexes (RuL(3), where L = CH(COR)(2)(-) and R = methyl, ethyl, n-propyl, n-butyl, isopropyl, and tert-butyl). Values for Delta G(i) degrees and Delta G(a) degrees were determined from electron-transfer equilibrium measurements by using Fourier transform ion cyclotron resonance mass spectrometry. Substituted benzenes and metallocenes were used as reference compounds. Cyclic voltammetry was used to determine E(1/2) values, which were obtained in N,N-dimethylformamide and measured relative to the ferrocene/ferrocenium couple. Substitution of methyl with larger substituents results in cathodic shifts in both oxidation and reduction potentials, and the solution data correlate well with the sums of Taft alkyl substituent parameters (sigma(I)). Gas-phase cations and anions are stabilized relative to the neutral by larger alkyl substituents, rendering Delta G(i) degrees less endoergic and Delta G(a) degrees more exoergic as the alkyl group size increases.